I hate them DM-201s... If I have good melee damage, I like to use a stone of blink to get next to them, then just whack them. They're tough, though, so having a potion of healing handy is a good idea.
Not that rare for me, unless it's a seed thing or an ascent thjng. In fact I've had some runs where I've even gotten two in the same room.
They're maybe a little less common in my runs than Golems are in the Metropolis levels.
I usually don't have too much trouble with dispatching them, although my favorite ways are to knock them down chasms. Not as much XP, but itnsaves on health pots