When you slay the makron, do you get a star? or only in challenge mode?

Of course it's only in challenge mode. The Makron has already been slayed many thousand times in normal mode since COOP or DIE exist

so how do i prove that i've killed him?

All you can see in the webchat at top of the website (and a little more) is stored in a file :
viewtopic.php?p=976#p976

I'm in the way to rewrite the COOP or DIE player's guide. The challenge mode will be better explained.

The basic rules are :
- Go to your profile and turn your account into challenge mode. You need to die one time for resetting your inventory.
- Play alone or only with challenge mode players, no help or item share from regular players.