Alright, 5 man mission complete. You were the first friend I took Angelwatch, but I failed my first run, and you never showed up again, so had to do the clears with random Terras. For those who care, here's my 3rd kill video with a 5 man team (no healer!):
Skeleton King - 5 man kill
For the 5 man team (hardest mission), I did iNichol, Soleil, Minfilia, TTerra, friend TTerra. I kicked Tilith off the team because honestly, with Illusionist Nichol, who needs tanks or healers am I right? Anyhow, here's some tips I've observed from my various kills:
If you can evade tank + Nichol, you can ignore all physical damage and not bother breaking his ATK.
If you drain the bosses mana, his magical damage is removed entirely above 60% health.
Spend time above 60% to fill up your limit bars, do the esper mission if you're doing it that run, etc. Once you push him under 60% it becomes a race.
After 60% the boss starts doing an aoe mana drain. It's not too strong of a mana drain (if you have good spirit to resist it) but it will slowly drain your team unless you're using a high end mana battery. Because he's draining mana, you can stop bothering to osmose him as it won't be very effective anymore. Boss starts doing some aoe fire damage, so fire resist helps here.
Once the boss drops under 40%, he enters his final phase and starts seriously hurting. You don't want to spend more than 2-3 turns under 40%, so burst him ASAP. His most dangerous attack is a massive AOE Shadowstorm (dark damage), and he applies a -80% dark debuff on your team. I had to gear my guys in mostly 40-60% shadow resist gear + Minfilia's +70% buff to survive it.
Anyhow, he's pretty hard, especially doing it with only 5 units, but the reward for a 5 man clear is mostly a prestige item. If you can kill this guy with 5 units, the reward is worthless to you lol.