How to Train Your Dragon

You can get this by playing 4v1 fights... you don't need to fight a 4-dragon team. (That'll make it easier right there!)

Once you get four dragons in the story, go into the arcade to the dragon select screen and hit your LB to import your dragons.

My suggestion is to do this after you've finished the game with a leveled-up Deadly Nadder. Basically mashing the Y button should mow through any opposition you pick... especially if you play on Easy and against a dragon with low stats.

Also, you may consider working on "Commander" while working toward this, since those wins will count toward your 20.

Just keep hitting Rematch and ignoring the hints to take a break from the game, and this should only take a little while.

Earliest this can be done is after the 3rd tournament as this is when you acquire your 4th dragon.

Go to the arcade screen select 1 player and pick your viking. When you get to the dragon select screen press LB to import your Dragons, then pick them all.

Win 20 fights (beat all 4 opposing dragons) with them. you can select rematch after each fight.