I've not often played wizards like I think I remember you are, so I don't have great wizard strategies.
As you have figured out, the cleric is your main target. Unfortunately, it's hard to get your henchman to target a specific enemy. What you can't do is try to fight them through the doorway. You need to either be in the room with them before the battle starts, or draw them out of the room to another area that is more advantageous for you.
If you have some invisibility available to you, you might be able to get you and your henchman (with a follow command) into the room, up next to the cleric, and then unload on the cleric. Of course that means the Chosen will surround you and attack you, so you'll need a plan to clean them up while surrounded.
I don't know if the cleric is using a spell or scripted ability. If it's a regular spell, you might have a shot at preventing his raising the Chosen by using a Silence spell, or something else that disrupts a caster's ability to cast. Anything you have to immobilize him may work.
You might be able to break up the Chosen by having them chase you through the dungeon. Take your henchman all the way back to the entrance, issue a Stand Your Ground order, and make your henchman invisible. Then go back to the Chosen, and as soon as the first one spots you, run and get them to chase. Hopefully not all will see you and chase. If you are moving faster with Haste, they should give up on chasing you as you run out of line of sight, and maybe stand around where they lost you. You might be able to string them out throughout the dungeon, so you can go back invisibly and take the cleric with fewer Chosen support.
Alternately, if you string them out, you can try taking out the Chosen one at a time. The cleric may not be able to raise Chosen out of line-of-sight. Better yet, if the cleric is scripted to go raise a fallen Chosen regardless of line of sight, you can use a stray dead Chosen as bait to lure him out of the room.