targeted by what? abilities?
awesome idea. if used with angels (targets card with healing) it could be devastating.
if targeted with momentum , does it ignore attacking creatures?
Targeted by anything that targets. Momentum allows creatures to ignore shields, but since Monsters never attack, that doesn't have any effect.
To take your example, suppose you had an Angel and a momentum:
You play a Monster, buff it with Momentum (+1|+1 and +2|+2 from Rancor), and then "heal" it with the Angel (no effect except getting +2|+2 from being targeted). It gains +1|+1 permanently, and +4|+4 from Rancor until the end of the turn for a total of 3|5 + 5|5 = 8|10.
At the end of the turn, it battles, lets say, a Black Dragon (10|5). They both target each other for battle. When it gets targeted, it gets another +2|+2 until the end of the turn from Rancor, for a total of 10|12. It kills the Black Dragon, takes 10 damage, and then loses the temporary +6|+6, dropping down to 4|2 (max HP 6).
If it lives until your next turn, you could heal it again with the Angel and get another +2|+2, which would put it at 4|6 +2|+2 (Rancor) = 6|8.