grumbel: I am currently in level 10 on the default Rookie difficulty and some parts of the game are completly insane even on that difficulty. The regular enemies aren't much of a problem, but so far I have not figured out how to deal with either the Supermechs (red robots that shoot homing missiles) or the Driller (silver robot with vulcan gun). Driller will often fire and hit you before the door has even fully opened and Supermechs don't leave you much chance to dodge their missiles. My current way of dealing with them is to load&save after essentially every hit I make to them and stay as far away from them as possible, but even with that and knowing where they are hiding I have a rather low success rate and have to retry far more then I would like to. Shield refills are also getting extremely rare.
The difficulty really doesn't feel right and I am really wondering if faster CPUs/Dosbox emu is screwing with the AI or something like that.
I believe the difficulty is very much in line with the original release, both running in DOSBox or with Rebirth. The following generally works pretty well for me.
The Drillers are the most annoying in my opinion though I find that using the vulcan cannon on them is highly effective since they seem to be somewhat stunned by the continuous stream of hits from that weapon. When you know where they are, you can often come around a corner or through a door guns blazing such that they don't have the opportunity to fire at you. Homing missiles can also turn corners to some degree so you can engage them sight unseen as well.
For the super hulks (red homing missile guys), if you're in an open area, you can reliably dodge the missiles if you're moving towards them and strafing horizontally and/or vertically, preferably all three direction at once. In tight spaces it can sometimes be a bit more tricky since if there isn't a door or terrain to hide behind, the missile might be able to track you around the corner somewhat. Those cases are definitely more tricky. The key in general though is strafing and delivering a lot of firepower quickly and accurately so they get at most one shot off which hopefully is dodged. The vulcan cannon has a similar effect on these guys, they can't really shoot if you keep it trained on them.
I'm by no means an amazing player, but I have struggled through the original Descent on Ace, it can be done! Good luck!