Tabuu is actually very easy once you understand his attack patterns. His OHKO is actually time-based in 45 second increments. The first OHKO unavoidable death happens at 0:45, the second happens at 2:15, the third happens at 4:30, the fourth happens at 7:30, and so forth. This means when selecting your character lineup it is imperative that you put your characters with higher-damaging aerial moves later in the lineup, as your first character will unavoidably die at 0:45, your second at 2:15, and so forth. The rest of his moves are easily avoidable. You should not lose any of your characters prior to his OHKOs. If you do, it significantly lowers your chances of completing the fight.
Ideally, your 4th, 5th, and 6th characters are your primary damage dealers. Using your 1st, 2nd and 3rd characters as your sacrifices gives you some leeway. If you are confident that you can keep your first character alive until 0:45 and your second character alive until 2:15, then you can put one of your damage-dealers in as 3rd in the rotation, with the understanding that he only has 2:15 to get his damage in before he is killed.
All told, it reminds me of Vaelastrasz from WoW pre-BC.