The best and least stressful opportunity is against the Golem boss. He has three main attacks: his ground spikes, his spawned crawlers, and the polarity rain. The only tricky part is knowing when to jump off his hammer; watch the ground spikes as they travel away from the Golem and return. His hammer will flash which is your signal to jump off. Crawlers can be taken care of by heading close to the Golem and sliding to the left until they are all dead. If you've already killed the Golem and don't want to start over, arcade mode works as well.