There IS a way to pseudo-program this.
In the event section below the monster group, (let's use Blob as my example monster) set an event. When Blob's HP goes below (example), 35%, the event starts. The first part of the event goes to the set variable section. In this set variable section, in the first box, click the 3rd circle (varibl) and set the variable as a new variable which will be the monster's hp. You can do this by clicking the 7th circle down. (I'll call this variable A) Next up, kill Blob via an event and summon a new monster. Blob2 shall be my example. Then, should Blob2 die, summon a Blob monster, and set hp = to variable A. This can be done via the Change Enemy HP button in the events bar.
Okay, now for the graphics part of all this. This is actually fairly simple. You can...
A) Start a battle with said monster, then take a screen shot and make that a back drop. Then, use the back drop change command to make the monster SEEM to be beneath him.
B) Replace the monster with a monster that has max HP, max DEF, max AGI, max MIND, and his only command is wait. Then, make it so that he has a stat condition which will double his evasion.
C) Route from appearance the Blob and make a new monster sprite that LOOKS like the blob is behind it.