The sheep level wasn’t so bad. The sheep barely made it a third of the way through by time I killed him.
What you need is at least 2 high level knights (I used four at the first corner). I also used all 6 archers by the time he hit the first corner. Oh, don’t forget two ice mages.
Put one berserker right where the flood of sheep comes out. Give him a level or two (whatever it takes to insta-kill them) and set him to attack weak enemies.
Place one knight, one ice mage, and another knight right next to each other at the super sheep entrance so it looks like this (X=knight O=ice mage) X O X
Max out the first knight. Whatever scrap you have left can go into the next knight. The key here is the knights last couple of abilities which causes a ton of damage.
So basically, as the sheep passes your first knights range, recall him, add an ice mage on the next tile, and make sure your second knight is pumped up in levels and ready to go.
As time goes on, your berserker will be racking up more scrap for you then you are spending on leveling on the knights and what you are losing on recalling them. With this extra scrap you can buy archers and level them up. Also, when the sheep reaches the first corner you should be able to add more knights on the other side to hit him.
It’s a bit of a time consuming process, but it works flawlessly.