https://github.com/elBukkit/MagicPlugin/blob/master/Magic/src/main/java/com/elmakers/mine/bukkit/magic/Mage.java#L567
When I have dealt 400+ damage, the attribute damage_dealt is 5 (because I'm holding a wood sword)
parameters suggestion

it returns 0, can you implement the feature that get values from papi placeholder in actions but not in parameters?
it's useful when I have many modifyvariables

The next dev build will change the behavior of the damage_dealt attribute.

That placeholder should work. I can't test it myself now, but looking at the code any parameters should pass through PAPI. I'll leave this open until I can check, but it may not be anytime soon.