Optional bosses are always good if they provide a variation to tactics/gameplay, or to the way you actually approach the mechanics.
Two good examples are, firstly, the optional bosses in World of Warcraft. Tempest Keep comes to mind. Fighting them usually takes a different strategy than the customary tank-and-spank of most of the others and is often more challenging than the normal run.
Secondly, as to the twisting of the mechanics, Borderlands 2 is a good example, with Vermivorous the Invincible: Fighting Vermi isn't that hard. It's tedious, mildly annoying, and not that much of a challenge since he telegraphs his direct attack and his indirect one takes a long time to actually hit and can be intercepted.
What's tough is actually getting him to spawn in the first place, which is a rather daunting, time consuming, dangerous process. The fight is getting Vermi spawned in the first place, not killing him, whereas most bosses are neatly placed in the world, pre-spawned, and are patiently waiting for the PCs to come along and do that thing they do so well.
Two good examples are, firstly, the optional bosses in World of Warcraft. Tempest Keep comes to mind. Fighting them usually takes a different strategy than the customary tank-and-spank of most of the others and is often more challenging than the normal run.
Secondly, as to the twisting of the mechanics, Borderlands 2 is a good example, with Vermivorous the Invincible: Fighting Vermi isn't that hard. It's tedious, mildly annoying, and not that much of a challenge since he telegraphs his direct attack and his indirect one takes a long time to actually hit and can be intercepted.
What's tough is actually getting him to spawn in the first place, which is a rather daunting, time consuming, dangerous process. The fight is getting Vermi spawned in the first place, not killing him, whereas most bosses are neatly placed in the world, pre-spawned, and are patiently waiting for the PCs to come along and do that thing they do so well.