Bite plus mist conversion and coven and a lot of mana and a lot of minions but you need some important items.
1. Forest Band 15% chance to heal army for 250hp. you want to fight him in creep waves so 15% happens often.
2. Vlemish, Plenor, Theurgist (reduce his hp/sec and increase minions health) Mist will set this off.
3. Platemail More minion health.
4. minitors, demolishers and bishops
5. Horn of Battle offsets his ooze damage to minions.
6. Most important item Magnus Rod. Launch rod bite then mist reduced mana cost for 5 seconds. has a 15 second cooldown rinse and repeat.
Beast will not stay and fight if he fights you in your mist he is stupid. You are healing your minions with mist, Horn and forest band.
And generating new ones if you stay in creep waves.
If you can upgrade to Hungrilings ASAP from Theurgist also so you can take advantage of 25% mana cost reduction this and Magnus Rod allow you to Mist a long time.
3 Helms Hung, Vlem, Plenor and Magnus Rod = 84 Mana per second. 25% reduction in Mana = mist costs 94 Mana per second.
At level 10 you would have 5000 Mana or you could mist for 8 minutes no hp stacking beast will stay and fight your minion horde for 8 minutes without potions or going back to citadel