Two Dire Flan just merged into Monstrous Flan.  The thing has over 200 thousand hp.  I'm scared.
 
More on topic, at this point in the game, I still find flans, particularly ones that come in large groups, to be the most difficult to handle.  I have to be thankful the yellow flans don't merge as fast as their green counterparts, but once they do, it hurts like all hell, and sentinels don't do much to help, giant aoe attacks and all.  Any enemy type or combinations got you playing with paradigm possibilities more than you imagined?

A large majority of the Flans are easy to preemptive strike where they become complete pushovers. Just use the invis stuff or find a good time to sneak up on them. 
 
As for my personal experiences my main damage groups were com/com/rav for a long time on single enemies, it wasn't until I realized how fast ravs can really move the stagger meter until I made rav/rav/com a main part of my setup, makes me feel pretty dumb looking back to earlier chapters when I had 3 chars but neglected to use that setup.

The "Flanitors" that heal each other with ambulance sirens cracked me up so much.  One of the stupidest enemies in any game. 
 
God I love this game.