Onslaugh at least one level is handy - mostly you can have fast units at the nearly start of the game (lake dragonflies) but the enemy can have them as well - so to be first in the battle is quite handy.
Frenzy is the spell that changed always the scale of winning in my favour (mostly when enemy is casting spells like clone/phoenix, ressurect, but most often when fighting bosses who spawn tons of small units, or when fighting the undead who have necromancers - more often they reanimate dead stack better for me, my army got +6 attack each time with easy kill of reanimated units and it was quite fun to see how the enemy tries to overhelm me but is actually making it each turn more easy for me
