I don't mind really tough battles that require skill and strategy to overcome (even if they're long). They can be frustrating but the payoff is that you feel damn good when you do make it through.
The only time I considered a game unfair was in Final Fantasy Tactics when you fight against Velius. The fight is spread in two sections and the game allows you to save between those two fights but if you do you're forced to win the next battle because there's no backing out at that point. It almost ruined my save file the first time I played because that last fight is a bitch and I couldn't leave and level up. I eventually won the fight after many tries, countless adjustments to my team and some luck.
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