Looking at the low sub-tier combat primarily:
To be fair, it's a moderate difficulty encounter, so being chewed up is the expected outcome for the baddies here (especially considering that despite previously routing from the eagle knights, they're now supposed to fight to the death instead of using their invisibility to sneak past the PCs). In addition, they have a horrible position, waaay to close to the starting point of the PCs considering they are both ranged combatants (I have no idea why a "sharpshooter" with a 120 ft. range increment crossbow and darkvision would choose to pick a position this close to the enemy instead of waiting somewhere in the darkness >200 ft. away and actually sniping at those PCs who do not have darkvision - imagine being in a cave seeing no further than your torch can shine and every now and then a bolt comes flying at you out of the dark! Yes, it's slightly elevated, but not high enough to actually stop a dedicated assault force, i.e. the PCs, from just rushing the mound.
Best strategy I can come up with (low-subtier): focus fire on one of the low-level, low-AC, low-hp back-rank squishies, ideally bringing one of them to dying. Then keep throwing bombs at that dying PC, hopefully hitting him several more times (bombs do splash damage even on regular fails, and even a little damage while dying increases your dying condition by one each time.) Get the PCs really worried about their friend's life, tying up some of their resources. PCs will usually outnumber NPCs in this fight, so you need to even the odds somewhat. If it looks like your position is getting rushed, cast invisibility on yourselves and reposition - ideally move any sharpshooters waaaay far off to make use of their really, really long range! Don't wait too long with the invisibility, it's your best bet to improve your currently untenable position! In fact, it might be a good idea to just do this as the very first thing, though I'd consider trying the focus fire at least for a round. The invisibility lasts 10 mins, so take your time to reposition, let them wait for a few rounds, give them the chance to do something dumb, let spell durations run out, rage wear off, etc. and then do the same again, focus fire on the next target. When the alchemist is out of bombs and infused reagents (assuming he is still alive by then) cast enlarge and use warhammer as a last-ditch attempt.
If you have multiple sharpshooters due to scaling, your odds are improving slightly - again, use your range and focus your fire.
In high subtier, you're at least getting a somewhat decent melee combatant (though one level 3 enemy is not going to be enough against a high sub-tier party, obviously...). He has a number of useful spels, but again, being way too close to the PCs at the start, he's going to be rushed and not have the time to cast them. If you could get even one extra round to have magic weapon already cast that would be a noticable improvement. But as it is, you'll likely spend your first round casting that and in the second or third round, you'll go down...
The more I think about it, the more the answer is to cast invisibility asap. All Duergar. Then spend a few rounds all move actions towards the north, spreading the ranged in a quarter circle. Or if you don't want to extend the map (the part of the cavern shown is not even 1 range increment for the crossbow across!), head for all the corners except for SE. Then sneak up on one of the PCs with the melee and focus fire/jump him from invisibility (you can cast magic weapon and enlarge while invisible).