That's great! My enemy "A.I." I have to use quotations for so that'll give you a bit on how well it works haha
It's super simple on the main map- they just check to see where you are in relation to their X/Y position and move a tile accordingly (most of the time, which is by design in order to keep it a little fresh I guess). It's obviously super easy to fool and avoid a single monster however once you get a few of them in there plus the layout of the floor it can get tricky. If you are invisible it tweaks this a bit- it uses a percentage, so say if you equip a regular ring of invisibility that gives you 50% invis, and then before each monster goes to move it'll have the condition If random(100)+1 > invis_effect too. So having this ring on means half the time the enemy won't even move because it doesn't see you. The invis effect from a statue though is always 100, so they'll always stay put. The effect doesn't last long so it's not too OP.
The main tactics or whatever you want to call it comes from the 1 vs 1 fights of course. Each enemy has its own battle options that are loaded into a list object off screen and the game will randomly pull from that list each time its turn comes around. You can give them higher percentages of a regular attack simply by adding more "ATTACK" lines, it's so easy and works great. If an enemy has a "HEAL" option and it comes up it'll first check to see what it's current vs maximum HP is, so that if it doesn't need to heal it'll default to something else so the spell isn't wasted needlessly.
I like how you said the Spelunky method is quick and easy, yet almost nobody has been able to really duplicate it since or at least not very well. See that's the kind of stuff you'll be great at for sure.