Ok, not quite random either. Rather than changing based on how you attack it, the monstrosity gains benefits based on the chests you open. If you open the crystal chest, the next monstrosity will be immune to magic, if you open the flesh chest, following mobs will have more HP, etc.
The sphinx says it learns/changes based on what you are doing, but I think that's just fluff/flavor text.
Seems like the powerups are pretty much random to me. Did it once, and was fire debuffing/nuking the heck out of the mob until the very end, I think like 8 pops. That time, it didn't start flurrying until the very last mob, was magic resist immune starting 2nd mob, melee resistant by like the 6th mob. Never did generate fire immunity.
The next night I did it again, and the mob started flurrying on the very first pop, and was fire/ice immune on the 3rd spawn, essentially completely magic immune by the end (even panther resisted), but never seemed to develop any particular melee resistance.
Did this a couple of nights ago and got very unlucky on the chests. We ended up having to kill 10 or 11. By the last one it was resistant to melee and pretty much every type of spell we threw at it. We ended up have to damage shield it down.
Group makeup - All Anguish+
warrior (me) cleric shammy druid mage rogue
The damage shield method worked very well just long, just long. The cleric and shammy kept me healed while the druid and mage chain cast DS's on me. The rogue and I did what little damage we could.
*edit* the mob talk is lies. Mobs dont learn from you. The chests opened do indeed determine mob's new stats/effects. Mobs are still kiteable, if needed. (Snare/Root are still magic based from druid/ranger tho :P)