I fail to see how a 20% damage boost moves battle stomp from unusable to good.
The damage is too high, because in most encounters you can destroy armor and CC enemies turn 1 easily, even bosses.
I've played with a full mage party with only 1 summoner, it was perfectly doable, but you actually want loremaster to check enemy resistances and play accordingly. In some places I feel like resistances are bit overdone but overall it's not that bad. You don't need to respec your mages either, just make sure you cover all elements with your party as well as each mage having some proficiency in 2-3 and/or having some useful support abilities memorized when he can't contribute much damage.
