To clarify, cheap/unfair bosses are few and far between, but there are some.
Crystal Guardian 2.0 is dependent on RNG if you want to beat him legit (and not through stacking up your HP and damage output and facetanking).
Watcher Knights are mostly fair, but if they decide to desynchronize, or if one goes high while the other goes low, you WILL get hit.
Brooding Mawlek is not too bad if you wait for uogrades before killing it, but in a vacuum, one of his attacks can be nigh undodgeable.
This does not apply to other hard bosses, such as NKG, which can be brutal, but require reflexes and pattern recognition, and are therefore entirely fair. ---
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