I'm curious what level your units are at if they're simply killing everything without retaliation. I'm nearing the end of the GD route on hard (or so my friends tell me), and while it's nowhere near the hardest strategy game I've played, there's definitely a degree to which I need to be careful in order to not get assblasted immediately (i.e. can't just charge in recklessly, can't place Lysithea in cavalry range, etc.). I've given some units both magic/physical skills but one or the other just ends up not being that great and not worth using. Bows also wind up being really bad on later maps without proper investment in my experience (unless it's just hoards of flying enemies).
As for the monastery, I'd honestly recommend skipping it if it bores you. It kinda got a bit samey to me late game, and unless you really need to improve your professor level or level up a weapon skill level for whatever reason, you can pretty much just rest/battle through everything with no consequences.
Overall I've liked it a lot so far. Certainly better than the Tellius games/Fates imo, potentially better than Awakening depending on whether it sticks the ending/whether there are any truly obnoxious maps in the endgame.
