I've already gone through the process of prepping for the next big roadblock with my previous post. Fortunately, having already played through this with 3 characters I already know Dark Force 2's song and dance routine. Zie has 3 abilities: Shadow Breath which does a hefty amount of damage to one character, Light Shower which delivers mid-line electrical damage to the whole group, and Evil Eye which tries to put one character to sleep.
Of these 3 Shadow Breath is the main threat. See the previous post for the stats and math for it and Light Shower. Since its a factor 1 attack nothing provides resistance to it. Well, okay, there are technically pieces of armor that provide factor 1 resistance but due to the way the game is programmed this resistance only gets applied during a surprise round - which doesn't apply to bosses since they can't be surprised. It does, however, check against magic defense and although Rika doesn't have Barrier she can equip a Reflect Shield for another 16 points in that stat. (Out of all the characters, Wren and Demi are best-suited for this boss.)

I took Rika all the way to level 57 for this fight knowing full well that two or three more would more or less cinch it unless the Numan was put to sleep for multiple rounds. As it was she could win IF her luck held out as I played the odds on the Shadow Breath damage rolls. Putting Demi into my party in the first slot, knowing she would die immediately but be removed from the party in the cutscenes thereafter (normally that's when Kyra leaves the group), I swapped the Genocyclaw for the Reflect Shield, took a deep breath, and walked the few steps to the north where Dark Force 2 is waiting offscreen in the above screenshot.
As I said before I was literally playing the odds with this one. An extra 20 - 30 hitpoints would have practically guaranteed victory so long as I didn't run out of healing. That, however, would be less fun.

Seriously, not kidding about the risks. The 220 damage in the screenshot above is from a failed attempt. I used up ALL of Rika's TP healing, alternating between Gires to top her off if she was around 200 health and I wasn't feeling confident about attacking and Nares/Trimates if she had less than that. I ended up using one of Rika's 2 Sol-Dews near the end when she was left with only 9 hits points. Her max health sits right at that awkward cusp where Nares/Trimates might fully heal her or they might not depending on the RNG and running with such a tiny margin of error I didn't want to chance it at that point.
Evil Eye turns were actually best since, unlike Lashiec, the effect didn't always work and Rika stood a good chance of having a turn to attack before needing to heal again. I also knew she would need to inflict somewhere between 900 and 1200 damage with basic attacks so I got that out of the way early and kept general track of my progress by how many Double Slashes remained.

After a long and tense battle, always fearful that Dark Force would land sleep or a a high damage roll, the second incarnation fell with 1 Double Slash still in stock along with 9 Trimates and 1 Sol-Dew. Those long chains of nothing but Shadow Breath drain the reserves quickly!

Video Link: http://www.youtube.com/watch?v=KTzH4IVePEI