I think 8 is the minimum intelligence, and you can have 6 easily just with a garment.
If you have a warrior that lacks intelligence, self buffs are more reliable than most offensive and/or crowd control spells (due to %hit chance). But truly, many options are possibles and I wouldn't exclude those spell on principle alone. Boulder Dash (the one that knocks down) is quite effective.
Witchcraft has Oath of Desecration which is a 50% damage buff (8 INT) and Absorb the elements (9 INT) which gives 50% resit more and can put you above the 80% soft cap (so elemental damage starts healing you), Pyro has Wildfire which is a haste buff (8 INT) (Scoundrel also has this buff, for only 4 AP, but higher cooldown and lasts one turn less I believe). All elemental school have elemental shields, which are overpowered. Earth shield requires 9 INT, fire shield 10, and the two others 11. Earth has the 30% hit buff and the armor buff aswell (both 8 INT). Air has invisibility (10 INT) and a 50% physical damage reduction buff (9 INT). Air has teleportation.
Note that your warriors should already have stuff to do while waiting for the ennemy to come closer: they have buffs and long range crowd control spells (not to mention their charge / dive).
All schools are good, and they all have low level spells that are always useful. It's hard to properly advice you due to that. One thing that I haven't tested myself is a warrior with 10 INT and some points invested in the Fire school, to use an "Explode"-type gameplay. I already basically played like that, buffing the warrior's fire resistance with the mages (Fire Shield + Absorb the elements), burning everything near the ennemies and letting the warriors run through this hell; but not with the "Explode" spell.
