how many people can you recruit in your party?
Four people can fight at once, everyone else is in reserve mode. Keep in mind that you can swap people out even during a fight, though I haven't personally tried that. Only the ones that were in a fight get XP though. I haven't messed around with swaps myself to know how it affects tension...
That said, for the people that said that Tainy and slime-girl gem are meh.
I just have one thing to say. Tainy's Max Tension skill is Continuous Use. What does that mean? Her turn doesn't end after she uses it. The skill has no cost and cooldown. It also buffs her attack.
So basically, for every boss fight since I've figured that out, I've had Tainy with the best items and gems to boost her agility, and the Slime gem to boost her Tension gain... and then I win. That's it. Spam that skill until everything dies. The rest of the party is just there to help her reach that point.
It makes farming XP and items at a certain Altar brain-dead easy.
Btw. Did anyone try New Game+ to the end. Do you have any idea if a certain Final Boss' drop can be gathered multiple times through multiple playthroughs?