Wait, isn't that the (Arbiter) level where you're going around an alien landscape and the Flood are fighting the Sentinels with tanks 'n stuff? I love that level! In fact, that was my favorite flood-related level
by far. Serious. It had that grand sense of you coming in the middle of this huge, crazy battle. I loved it.
I heard that Regret was tough but not impossible on Legendary, while Tatarus, on the other hand, was infuriating. Unfortunately, I haven't had the time or the privilege to play the game on Legendary.