You'll have to park many of your favorite ColecoVision techniques at the door but you'll enjoy yourself doing it thanks to IntyBASIC.
Another trick you'll want to learn is used by Worm Womper for the many moving critters...they aren't MOBs, but pre-shifted GRAM cards.
[Click to animate]
This is from a long time ago so isn't optimized by any means, but shows the MOBless technique.
You can also mass change/animate large areas by DEFINE/DEFINE ALTERNATE a gram card or two like the waterfalls in the ZYX video I posted last weekend.
Another trick is plan ahead and break up your 64 cards into logical sets of stuff
0-9 mob characters for player (and the effectiveness can be extended by redefining cards on the fly, say walking animation vs falling, only one animation set needs to be in the GRAM at a time)
11-20 visible enemies (same trick as above)
In this case, limit your screens drawn in a paint program to 40ish unique cards.
Using IntyColor, you can make a bunch of different backgrounds, each with different card sets for slots 21-63, use the parameter -o21 when generating the screens so you won't have worry about interfering with the first 20 cards reserved for player and critters.