Board Thread:Game Help/@comment-28684741-20170515215009/@comment-26440657-20170515220228

You can access that minigame before mechanical bosses but not most of the enemies will spawn so you can beat part of it pretty early on. You will only be able to beat the full thing after you kill Golem since all of the enemies will only spawn past that point.

Spoilers:

To be honest, Betsy is the only hard-ish boss to beat there, in my opinion. You could also fill your arena with traps and everything to make it extremely easy, you can even wall up the crystal if you want, to prevent projectiles from hitting it, it won't be safe from enemies that pass through walls though.