After being reborn, Tristan knew one thing for sure: living meant output.
The bosses he's facing now don't have the means to counter Stealth, but that doesn't mean future bosses won't. To take a step back, even if the native bosses are a joke, there's still the gods to consider. In front of those terrifying entities, even a momentary lapse can be fatal!
He's not the heavily armored warrior from his previous life who could just tank the damage. If a god got a hold of him, he'd probably be snatched away on sight.
The appearance of Rain of Corruption gave Tristan more options in battle.
If the enemy isn't strong, he can just use Rain of Corruption and Shadow Dagger together to wipe them out as fast as possible. But if the opponent is tough, he can hide and cast Rain of Corruption, making the encounter far safer.