It seems that a level 12 Fertile season will increase all creatures maximum HP by 476 (or close). As I understand, Deep wound does damage and lowers maximum HP by 20% of the maximum opponent HP, but the health and maximum come back when the Deep Wound ends.
Well, effectively Fertile Season adds 95 damage to a Deep Wound, and with a guild lord (say 2000 HP because I have absolutely no idea) that's about 500dmg.
So, if you can lower the Guild Lords hp to the 5-600 range with two Grenth's balances while getting fertile season up, a deep wound could almost finish the job, maybe with a vampiric gaze or two thrown in for good measure.
Too complicated? Too situational? I think it could be done VERY quickly with 3 players, and still in under 10 sec with assassins. The limiting factor becomes time for Fertile season to go up. If that is prepped while the necs are lowering their hp, it could be done VERY quickly I think.
Matt
*edit because I just thought that Guild Lords may not be considered creatures

Anyway, what about Fertile season in combination with a deep wound and Fevered Dreams? That's a very good amount of AoE damage.
Matt