The second ability of
Greenwarden of Murasa is a triggered ability. It triggers with
Greenwarden of Murasa dies. Here's how that plays out:
1) At the beginning of the end step, you sacrifice the token copy of
Greenwarden of Murasa. The token does actually go to the graveyard.
2) The second ability triggers and goes on the stack. Nothing is exiled at this point, only when this ability resolves.
3) Priority passes.
4) When priority passes, state based actions are checked. (When a creature has more damage marked on it than it has toughness, State-based actions are what put it in the graveyard. State-based actions also make tokens in any zone other than the battlefield
disappear from existence.) The token vanishes from the graveyard at this point.
5) The Greenwarden ability resolves. When it gets to the part that says "you may
exile {this}", the token has long-since vanished. It cannot be exiled to this ability.
Sorry. You do not get the benefit in this case.
Happy Brewing!