I was having some trouble with this, until I read "True Marvellous" 's advice on the combos.
Win the final Championship with none of your dragons being KO.
This refers to the Thor'sday Thursday Championship.
Defeating Fishlegs with his Gronckle was difficult.
But I concur on how to use the combo.
-Night Fury Level 22
-Max out Speed for fast combos
-Max out strength
-Distribute the other points as you please.
-Be about 2 dragon distances from your opponent.
-Use the Y Y Y combo to stun him.
-Pause for about 1/2 second.
-Use the Y Y Y combo again.
-Repeat until you win.
I was trying the Deadly Nadder per the advice in the guide.
Just use the Night Fury the entire game
and arcade achievements.
Good luck everyone.
HOW TO TRAIN YOUR DRAGON ... 1000G