So far my strat is to go for:
skeleton -> learning -> prosperity -> calm -> necromancy -> grim reaper
wrath tends to not be needed tbh, you make enough units to oneshot the enemy anyways because the generation skill, like skeleton 5%, also increases damage anyways. Unless you have about 1.4k base defense stats or so which means calm isn't needed, in which case wrath is fine (not that it matters tbh lmao)
and only revive skeletons, then grim reapers when you get them. get just enough skeletons and grim reapers they can oneshot the enemy. I prefer to have about equal units to the enemy because it doesn't change defense/regen much.
Grim reapers are good cause they regen 1/20th the speed of skeletons, but have 20x health. So they're equal to skeleton regen, except they also create skeletons.
edit: going to investigate if zombies are better. they seem to have slightly higher HP per second production.
edit2: no, zombies aren't better cause they lack the grim reaper-skeleton synergy.