I hear you, man.
Chapter 21 of FE6 is horrible in that regard as well. It has a grand total of 22 standard reinforcement units plus one secondary boss (!) that appear out of seemingly nowhere over the course of the battle. Worst of all, these are almost all wyvern lords carrying silver or ranged weapons, which gives them incredible mobility and destructive potential. Absolute pain to deal with.
- Spoiler: show
Basically, if you aren't hiding Roy and your healers inside a makeshift roman-style tortoise formation as you progress, it's going to hurt. And no casual mode or battle saves to help you out.
(Still one of my favourite chapters in the series thanks to the writing and because of how absolutely insane it all is.)