General guideline:
For fighters, moon Bhaal into following you and lead him to Imoen where Bhaal's invulnerability is removed. Proceed to hack him to death.
For spellcasters, prepare Stoneskin beforehand, moon Bhaal, run to Imoen, summon as many summons as you can and let them tank for you as you cast spells against Bhaal.
For all, if you have any items that can summon creatures at that moment, use them.
For future games, before confronting Irenicus in the Asylum, soup up your main character with the best items and summoning tools, such as Boots of Haste, Staff of Fire Control, blah, blah, blah.