The atk buff is capped via the fact that it has a duration. 12 turn duration = 6 applications at best, for peak value of 120% assault.
No cap on def listed; doesn't seem like people have tried hanging around for 20/25/30/etc. turns without casting any buffs to see. It's that quandary of needing an enemy that can survive that long as well as not killing you in the meantime :/