A small amount of luck being involved doesn't make a boss cheap. Yes, almost all RPG bosses are random to some degree. But whatever the boss throws at you, there should be a way to respond to it/counter it/prepare against it. The existence of an attack combination that's 100% impossible to survive and which a boss might use randomly can make the battle completely up to luck alone, however. If there is no possible way to prepare for or defend against a random non-timed/conditional attack, then I'd say that attack is a bit cheap. And I wouldn't say that it makes a boss hard, since luck is the sole deciding factor.
A boss can be hard if it has an attack that's really hard to survive, like... if Omega Weapon used Terra Break randomly, he would be harder since it would require you to be prepared for a Terra Break at all times. You would either need to have a character using Defend through the whole battle, or keep your party under Protect and at full health during each of Omega's turns. There would be some luck involved, since Omega using Terra Break often would be difficult to handle, while him hardly ever using it wouldn't be too bad. But it would be possible to go into the battle, and come out victorious even if Omega would decide to use Terra Break very frequently. Thus the random Terra Breaks would be a challenge to overcome.
But a random instant-game over is just a reset button. It can mean that the player will require a greater number of attempts to win, but it doesn't offer the player any challenge to be overcome, since there is nothing the player can actually do.





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