Just rush it. Create some razor girls, summon your flag, pick it up and go straight for the merch-booth on the right, double-teaming one of the girls. Keep building razor girls with maybe the odd kill-master for good measure, head straight for the stage. From that point on you should either be soloing or double-teaming razor girls as you attack the stage. It helps if you've got the rock-block and a good beast on the call-of-the-wild summon, but some well-placed face melters should be good enough really. Oh, and If you need to call in any more units and you're nearly done, fire barons will get to you quick.
My strategies are (mostly)
- Try and stay out of the air, be on the ground double-teaming (Razor girls!).
- Carry the flag with you most of the time.
- Be aggressive, not defensive.
- Time your solos well - try and get as many face-melters in as possible, preferably while you're utterly surrounded.
- Oh, you get fans for killing the enemy hero too.
If you're struggling, you can always do the hunter challenges until you can summon a hexadon, and the hunter gives you an awesome axe too. If you're on the second drowning doom battle, you are in a position to find all of the solos too, some of which are very handy. There are some guitar strings specifically designed to help solo cooldown, too. Get those.