Crota. Requires almost perfect timing and 6 people willing to work as a team, even when you know exactly how to take him down. Enrage means you can't just be patient and wear him down like a strike or mission boss.
Templar is pretty tough too at equivalent level.
Other than that, I tended to find the bosses were ok, but if I was going to struggle on a boss fight it was getting mobbed by trash not the boss itself. Bosses are tough enough to be a challenge but not so tough as to be a frustration. Which is ideal for a casual gamer like me.
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Plus one, the fodder usually gets you before the boss. Nothing sucks more than getting melee'd in the back by a dreg while laying into a boss with the light switch modifier active
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Ah yes, light switch. Good call. Thrall are a nightmare for that also.