I've played Phantasy Star 4 a few times. I've also soloed it a few times. (Once each with Chaz, Wren, Gryz, and Rika, respectively.) All those characters are fighters, however, and I've long wondered how some of the more fragile team members would fare. Well the best way to find out is to go ahead and play such a game.

So that's what I did. Here's Raja at level 1. Normally it would impossible to ever have him in the party at such a low level but I'm using BNC's party select hack plus my own shop modifications. In this hack every character starts at level 1.
So let's talk about Raja a bit. He's an old Dezolian priest. That means he gets lots of healing techniques (most of which will never see use in solo play), has fast Mental growth, and gets some good old fashioned holy powers. On the other hand being a priest he has the same equipment options as Rune which is to say that there's not much he can use. He can only wear the lightest armor and shields and his weapons are limited solely to staves. His starting stats are a good indication of what is to come.
Strength 6: Raja is not much of a melee attacker but he can do some damage with a weapon equipped.
Mental 12: Raja has perhaps the best Mental growth in the game. His Res line techniques will increase in power quickly and his magic defense will always be top-notch.
Agility 4: Raja starts with an Agility of 4 which is pretty bad news. Agility determines turn order, fleeing success rate, whether some status effects take hold, is factored into the character's Defense Power, and whether the party is ambushed by monsters when an encounter is triggered*.
Dexterity 3: Dexterity controls accuracy and critical hit rate in combat and might (with an emphasis on the might) evasion rate.
Combine this with the fact Raja starts the game with 17 hit points and no armor equipped making his Defense Power equal to his starting Agility and this is a recipe for getting killed a lot.
(*At least, I'm fairly sure that is the case although I'm not certain on the matter. Most characters will end up with close Agility and Dexterity scores which makes it hard to tell.)

And by 'a lot' I mean it took something like twenty tries just to win his first battle and get even a single experience point. Most battles started with Raja being ambushed and killed before he ever got the chance to even input a command.

When he did get a turn his only viable option for survival was to use a charge of his Blessing skill. Blessing is unique to Raja but it does the exact same thing the Deban technique does. It raises the entire party's defense by the user's Mental stat for the duration of combat. This is why when Rika and Raja are in the party at once it is better to have Raja use Blessing because his Mental will be higher then Rika's.
If Raja didn't start with Blessing this would be impossible because there's no armor for sale in Piata and he otherwise wouldn't survive a fight with even the weakest enemies in the game.
To get even his first level up Raja needed several Xanafalgue x2 formations where he was not be ambushed and survived to put Blessing up. Zoran Bults easily killed him and the Xanafalgue x3 formations usually overwhelmed with sheer numbers.

As it was if he could survive to cast Blessing he stood a good chance of winning.

After a great many deaths Raja slowly ground his way to level 4. With Blessing he had no problem against Igglanova whose critical hits could never do more then 1 damage. It was still a long fight (3+ minutes) and Raja had to heal several times but it was not difficult.

Raja could have beaten Igglanova sooner but I knew he was going to have a very rough time once he hit the overworld map and wanted a bit of a hit point buffer. Raja had to embark on the same time-honored tradition that my previous soloes endured. He had to make the Piata > Miles* > Zema > Molcum > Krup journey. This has always been one of the most frustrating parts of my solo runs and tends to rack up deaths. Even with my shop modifications there isn't much in the way of equipment.
Raja got to Miles after running from 2 fights, one was an ambush he barely survived. Once there he bought a Leather Band (helmet) increasing defense from 7 to 8. He then made it to Zema on the first try but there was nothing to do there yet with the whole town turned to stone. The run to Molcum involved many deaths but a few victories (at 100 - 250 xp per fight). Raja made it there at level 5.
Raja then made the short trip from Molcum to Krup. There was no new armor in Krup but he picked up a Telepipe in case he needed it later. Then I started a long grind just outside of town.
(*the name of this town is actually Mile in game but I'm leaving as-is in this report since I noted a correction with a later character)

Raja would like to make a public service announcement at this point that at level five sans armor locustas friggin hurt! That's an 18 damage non-critical against Raja's maximum 29 health at that level.

In that particular fight he got to demonstrate another cause of death. Raja got Blessing off but he's dead anyway because he's not fast enough to heal himself next round and enemy attacks always do a minimum of 1 damage if they hit (they always hit Raja with his low Dexterity). Yup, low Agility is definitely working against him.
After many more deaths Raja reached level 7 and was finally able to take down a Locusta x2 formation. At level 8 he learned Gires but his high mental meant that Res was still healing more then his maximum hit points per cast.

At level 9 Raja picked up another skill unique to him: Holyword. Unfortunately it would be useless for some time. When Raja reached level 10 he was consistently killing everything he ran into around Krup provided he wasn't ambushed. I felt he was finally strong enough to advance the game's plot a little bit so I hiked back to Zema to have the scene with Professor Holt then returned to Molcum for Rune to officially enter the game's plot.
Then it was off to Tonoe. Raja opened the Valley Maze and inside was immediately wasted by a 3xBlob ambush where the first Blob criticaled Raja for 32 damage. Ouch. Well it was immediately obvious he was not going to be fighting his way through the dungeon. Instead he was going to have to run as fast as he could to Tonoe.

He reached Tonoe after a couple tries at level 10. The town offered no new equipment, however, and several attempts to go straight to the chest with the Alshline in the Basement were met with inglorious deaths. I returned to grinding out on the overworld and the Maze. I actually didn't stay in the maze for long. The formations there yielded slightly more meseta but the experience wasn't worth the greatly increased risk.
Fun Fact: Carrion Crawlers have an Agility debuff called Thread. Thread is the only stat debuff that works on androids. In the vanilla game there is no reason to ever bring an android anywhere Carrion Crawlers appear in formations. Wren was the first party select hack solo I did and was a big prompt to adjust the game's shops because he also doesn't start with any armor and none appears until quite a way into the game. This was not a fun dungeon for him.

At level 13 Raja got the Alshline on the 3rd try by running from all fights and going straight down. He Escapiped out, then Telepiped to Zema.

The second Igganova was even less of a threat then the first and didn't require any heals at all, just Blessing. This fight would be more interesting if it was a Guilgenova (which appears in the Bio-Plant!) rather then another Igglanova. That would be a nice warning that the next dungeon is about to change things up.
After about a dozen deaths Raja finally managed to sprint his way through the Bio-Plant. If it seems like I'm skipping chunks of game that's because I essentially had to. Raja's special abilities don't amount for much here and he is fragile. When I say fragile I mean FRAGILE. It took little time for him to rack up more deaths then all my other solo runs combined - or at least it certainly felt that way since I don't keep track of that. By this point in the game I already knew he would have long periods of grinding things he could manage fairly safely and spend the rest of his time running frantically to reach his next objective without being slaughtered (too many times).

With the Bio-Plant behind him Raja bought a Circlet and White Mantle in Zema. He could have done this before the Bio-Plant but I figured it wouldn't make much difference in his survivability in there. However I was going to need whatever defense boost he could get because Raja was about to hit another grind wall and I knew it. He needed to make the dangerous trip to Aeido then from there to Kadary.
After a dozen more deaths Raja made it to Aeido.

He was level 13 at that time. Could he make it through the cave connecting Aeido to Kadary? Hahaha, hell no he couldn't.
Nalya is just east of Aeido so Raja went there instead. He could kill the Scorpirus x2 and x3 formations around town although they were long fights that drained his tech points. He was also easily poisoned by them due to his low Agility but poison doesn't do anything in battle and Anti is 2 TP so that at least was never an issue. Raja ground to level 14.

Then he headed into the Wreckage by Nalya. After many deaths Raja finally actually won a fight against the machines! The Whistle formations were risky but worth the xp. Warren286s however were super deadly. They have high Agility for that part of the game and Flare Shot, which, as the screenshot of Raja being shot for 46 damage shows, was nothing to sneeze at. Despite the name Flare Shot is actually a factor 2/energy attack, not a factor 3/fire attack but it was still pretty much instant death if Raja was ambushed by a formation with Warren286s in it.
(Seriously, Flare Shot of all things! That attack is usually a joke in the vanilla game.)
Ultimately Raja didn't go farther then the first hallway in the wreckage. The entire dungeon is optional. When he reached 15 I decided I was going to have to avail myself of a shop change. Without it Raja would be forced to keep on grinding on weak, boring enemies until he was massively over-leveled enough to sprint his way through the next section of the game. The game was never designed to do what I'm doing and part of what I'm doing is intensive testing to make sure any potential party formation is viable.
With that said I returned to Aeido and bought a Force Cane in the market for a very much needed damage boost. In vanilla a Force Cane does eventually become available but there's the little problem of Rune not being part of the travelling party at this point so there's no weapon upgrade for him or Raja until a dungeon down the road. That dungeon is not far off but of course 'not far off' is not now and Raja literally has no way of dealing damage outside of attacking. Said damage with the Wood Cane is laughably weak and getting worse with each increase in monster power.
A Force Cane has an attack power of 36 and gives +1 strength. For comparison in two-handers at this point in the game the Ceramic Sword offers 43 attack power and the Struggle Axe offers 41. A Saber Claw offers 18 but is one-handed and there's pretty much no reason for Rika to not dual wield claws right now. For a bit of added perspective on Raja's weapon selection he won't find a weapon with a higher attack power until a dungeon at the end of the game. Staves are in general pretty piss poor offense.

Anyway to make a long story short at level 16 and after numerous deaths Raja made it to Kadary. He still hadn't broken 100 max hit points.
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Gires/Nares can overcome a lot.