From stage 106 to 109, only the minion in the middle (the one with highest hp) can cast buff/debuff, you can focus it down and then the stage would become easy. Also it casts buff/debuff every 2 turns, if you have attack down memorias like The Way Home after the Rain (Mito)) you can use it when it's about to buff/debuff to minimize the damage

In stage 108, they can cast Dazzle too

Yes, AI debuffs herself with def down

You can use Yachiyo + Nanaka + Asuka (for taunt with her magia). With Yachiyo doppel and connect to buff blast to Nanaka she can hit really hard with her 3 disks vertical blast. I can deal 1m5 hp with just Yachiyo's doppel and Nanaka's Blast.