[Twan]

In fact I haven't experimented a lot boosting only pretenders as there is a better way to boost AIs : giving them some provinces with
-recruitable powerful units (troglodytes, trolls, knights on a province with high ressources)
-immobiles mages and a lab or appropriate site (ie enter to summon bane + immobile d1 mage)
-known gems prod sites (in provinces owned or nearby provinces)

Then you may let AI design the pretender, even if the choice is strange it will be hard.

(but I've tried one time to duel an AI just boosted by an immortal E10 cyclops, and I think it's one of the best boost if you just change pretender -but you may add nature and fire of course; give it relics etc-)