You need at least 5000 coins total for this, depending on how many tries you need. When you have a level 4/5 dragon or higher, select his habitat and select him. (now you're in the same screen as where you would've fed him) Now select train dragon, which costs 200 coins at a time. Now vikings and mermaids will be thrown up, aim for the vikings and don't aim for the mermaids. For a flawless training session, you need to hit all vikings, and save all mermaids(not hitting them). The higher level your dragon gets, the more vikings and mermaids will be thrown up, so it is best to do this with a low level dragon. Training starts from around level 4-5. It is best to hit the vikings at the time when they've stopped rising, and are about to start descending, since this is easiest.
-You don't need to do it all in one run.
-When you have had a flawless dragon training, your dragon will be champion(increased damage and HP) for 2 hours. You can still keep training him, for progress towards the achievement, but the amount of damage he deals and his HP will remain the same.