My players are still in the early stages of the campaign. So they haven't met the BBEG yet. Just his minions.
When they do meet him it will not be easy. He is an evil cleric (access to evil spell lists). Spell lists are Summons, Necromancy, Dark Channels and other TBD. Typical attack will be as follows:
- minion fighter/s defend from physical attack
- one minion cleric protect and heal
- another minion cleric insta-cast Summmons
- minion animist insta-cast "Animal Tongues" onto BBEG. So he doesn't need to concentrate on summoned animals. Next round maybe another "Animal Tongues" onto other summoning cleric or summon animals (or plant growth) with back up healing
- BBEG (lvl 16) insta-cast "Summons X" in Rd 1, then "Demonic Gate" in Rd 2 (since he doesn't need to concentrate on the summoned animals (Animal Tongues)
Remember all the trouble you have when fighting wolves (and other animals). Well, he can insta-summon and control 3-4 wolves or 2 lions or a large bear (or similiar). Summon them to appear directly behind the enemy casters so they get rear attack for the first round...
... and then gate in a Type I Demon...
... and then the fight will get serious!