Nothing like rout enemy maps with reinforcements...
"okay I've surrounded the boss, i'll kill him next turn and we're done"
*reinforcements*
"Boss defeated, all but one reinforcement dead, next turn it will truly be over"
*more reinforcements almost successfully cheapshotting Lissa to boot*
"OKAY THIS TIME IT'S ACTUALLY OVER RIGHT? RIGHT! GOOD!"
I've got to say, this is the only part that's really vexing me. You almost never know from where, nor do you know which kind of enemy type will appear.
I'm playing on hard and classic, all my deaths except for one were due to random first reinforcements that appeared.
Now, I learned for the most part, always using my healers as support if reinforcements are soon to come so they can't get focused, and just generally playing a lot more careful, like waiting to advance and just stand around for a round or two to wait if more reinforcements are coming.
None of that is enough though. Just this game Libra and one of my Pegasus Knights were killed, because the third reinforcement wave was not as expected more heroes and knights, but in fact a couple of riding Archers. They rode up, one-shotted my Pegasus Knight and the remaining three or what mercilessly focused on Libra, the only one I didn't support because I used him for some heals. I had put them outside the range of a hero and waited whether or not more reinforcement would come. But I wasn't expecting any riding Archers. Voila, dead.
It's just so frustrating and makes you feel utterly powerless. The rest of the fight was a piece of cake, more or less. Then reinforcements appear, move in and kill someone. No chance to react, no chance to do anything. No matter how hard you plan. Fight took the best part of half an hour too. Last 5 minutes two healers get randomly killed. Bah.
Rant over. No Fire Emblem for me anymore today.