|Rat Bastard DM|
If the module is fairly free flowing in how the enemies can be encountered, perhaps then its time to go a step beyond merely mechanically wearing them down. It may be time to wear them down psychologically with mind games, oh delicious mind games. You're already using disguise self quite a bit; use that to your full advantage to divide and conquer. Have a mook disguised as the cleric show up a couple of/a few encounters in to make them pull out their best resources prematurely and then become thwarted as they find out it was a ruse. Have disguised mooks run in opposite directions around corners and lure them into ambushes. Have them yell false instructions to each other to throw the party off to their tactics. Anything to make them harried; if they get flustered enough they might be thinking in top form.
Since your cleric's been scrying, she should have gotten a few glimpses of some all those minions they have. If you have enough lead-in time, have them abduct a less often seen NPC (perhaps one of their scouts in the field?) and then place the poor fool in an overly elaborate yet not-so-easily-escapable deathtrap (if the area has one) to split up the party and/or take up actions. Or barring that, there's nothing saying you can't disguise your own mooks as their own mooks to lure the PCs into disadvantageous situations. Mix up illusions with the real McCoy for best results.
Above all, you need some incentive to keep them inside the base. If your party has some timed objective (or at least, seems like there is some timed objective), then that might make them less likely to teleport if things go sour. Your cleric has sending, for example. You could just play with that without actually having to use it by leaving some papers (letters, etc.) lying around indicating a greater force is arriving shortly, or some horrid ritual is about to be enacted.