>>528966764
There are quite a few ways we can go about this.
Kaboom and Vault are good choices, only need 1 star.
The BP Gameball is also a good option, although it has very weak base attack, making it harder to reach maximum buff. But if you were going to level up the gameball anyway just to get another useful tool in your arsenal, this might b the way to go.

Finally Anton's engine also has ER. If you're someone who leveled up Anton+Sig, but no longer use him, you can transfer his Engine to Panda.