I finally completed al FEH maps on Abyssal. So I want to share my tactics to defeat some of them. In most cases it works to deploy Xmas Altina, because she easily defeats all Dragons and Mages, and boosts her Attack/Res when she has a penalty. Besides she equips DC and her blade always strike twice.
Brave Ike is also a great unit to use because he can act as a tank, blocking most attacks and then he recovers health with Aether. I also gave him DC and Null-C-Disrupt for further versatility.
In addition to these recommendations, I always try to use units that can defeat enemies on the very first turn. It is easier to handle reinforcements this way.
So, here are some videos from my channel, all of them are abyssals:
Consoles Universe 