As it is a creature, Esper Sentinel lets you alter its power and toughness with +1/+1 counters, enchantments, and artifacts
But thats three out of a hundred cards
(Character cards have to wait a turn before taking an action, but the other card types come right into the action.) When sung by the appropriate charactercurrently tied to the cost/value of the singing character)these Song cards enter the game at no cost
From MH3, there are 24 retro frame rares (28.4%), including fetch lands, 8 retro frame mythic rares (5.7%), 2 retro frame new-to-Modern rares (4.6%), and 1 mythic rare (1.1%)
Use it to remove an annoying opponents threat or gracefully retire one of your own battered characters
However, there was a clever Aura in the set which could work around that, letting you stack several spell effects onto a single creature and re-use them