OK, here is everything you need to know to do this achievement.
Ultra Combos can only be performed when your opponent has a flashing red health bar, you have a flashing yellow meter and you are at least one hit into a combo. Their second health bar is red, and it will flash when at 15-20%. Your yellow meter will fill fastest when you're taunting or blocking. You need to perform one move before you initiate the ULTRA combo.
Here's a walkthrough with Sabrewulf since I learned how to play that character while trying another person's lousy advice.
Go to Arcade mode with 2 controllers. Choose Sabrewulf for player one. Once in arcade mode, you have 99 seconds to deplete your opponent's life to a flashing red bar. At this point, use the combo
,
+
with Sabrewulf. This is his taunt, it will fill his meter until it's yellow and flashing.
Once the conditions are set, use the combo
,
+
, this will initiate a combo that looks like a cartwheel. Quickly enter the Ultra Combo NOW, (HOLD)
,
+
. If done correctly, it will show a cutscene and you will get an ULTRAAAAAA COOMBOOOOOOO!!
You can practice the sequence
,
+
, (HOLD)
,
+
while depleting your opponent's health bar. If entered correctly, Sabrewulf should do a cartwheel, then do a yelp/howl or something.
Congrats on your achievement.