Basically just kill the greeviling camps that spawn (last hits count). First team to kill 11 camps wins. The whistle allows you to change into a Greevil with random abilities (or preset ones if you have your greevil courier equipped). Everyone levels the same/Avoid picking invis units...Necrolyte for easy last hit steals (his ulti does percentage based damage allowing you to last hit earlier than anyone else can).
Ah okay. Got a few hints from a guy too about if you are low health try to flute it up to switch to greev.