Marvel Avengers Battle for Earth

Just find two characters you are comfortable with doing all of their Super Attacks with and then keep trying.

I got this with Black Widow and Scarlet Witch. I started with Scarlet Witches Launcher attack > switch to Black Widow and did the attack with your two hands straight up (Shield Attack) > then I'd do Black Widow's Slide Attack which is also a Launcher, so it would keep the enemy in the air a little longer > then Switch back to Scarlet witch and I'd do the Shield Attack with her and that would be a 4x Combo with switching characters twice.

Hope this helps.

UPDATE: Found this way would work as well.

Here's how:
1. Load up Versus mode.
2. Select Incredible Hulk first, Spiderman Second for your characters.
3. For the opponent, choose Black Widow, and whoever else, the second secondary character doesn't matter.
4. Start the match.
5. Start with Hulk's Leftmost Launcher Super Attack, and then combo switch between the two characters doing the left most attacks until you reach the end of both character's super attacks. You want to do this in one Combo and it'll register as a 5 hit Ultra Tag Combo, resulting in huge bonuses of XP.
(SA=Super Attack)

1. Hulk Left SA (Right Arm in Air, bring straight down),
2. Left hand up in air to switch to SpiderMan,
3. SpiderMan Left SA (Both arms out in T-shape, push towards middle), Tag Combo Hit #2
4. Left hand up in air to switch to Hulk,
5. Hulk Middle SA (Both arms out in T-shape, push towards middle), Super Tag Combo Hit #3
6. Left hand up in air to switch to SpiderMan,
7. SpiderMan Middle SA (Both arms over head, lift right leg), Ultra Tag Combo hit #4
8. SpiderMan Right SA (Right Arm in Air, bring straight down), Ultra Tag Combo Hit #5 :: Combo ends here.
9. Left hand up in air to switch to Hulk,
10. Hulk Right SA (Arms out to right side, swing like bat towards center)
11. Black Widow's HP is depleted, Round 2 or end match and retry.

See the video below to see it in action, look at Round 2 for a perfect Ultra Tag Combo.

Obviously with a 2.69 Ratio on this achievement it seems tougher than it should be. Throughout the entire game I couldn't chain the moves between two characters correctly and had a tough time till I found the easiest place to do this and did it my first attempt. Not sure however if you can go straight to the combo tutorial or if you need to do the other ones first, either way there is an achievement tied to doing all of the lessons.

1)Go to Fight Modes
2)Select Challenges
3)Select Training
4)Choose Combo
Now there will be 4 combo lessons in the tutorial, when you get to the 4th lesson it will show you how to perform combos between two characters. Whats nice about this place is your opponent WILL NOT move and the times between combos seem to be somewhat longer than in a real match. Just remember key word is switching characters TWICE. Start with a launcher move (First Move on the Left) switch, perform another launcher move (First Move on the Left),switch, and then perform two more moves.