This achievement should be gotten as you are leveling up your characters. Some facts about the achievement that should help you.
*Each character has a max level of 20, so the higher your level, the easier this achievement might be to accomplish
*The ranged characters, Scarlett, haakon and Avalon, can help a lot with the Goblin Chieftains. They show up at 1-3 and 1-6 and are the hardest of the enemies you face.
*The Goblin Chieftain is really only vulnerable after he's made a brutal slashing charging attack, unless you use a ranged weapon.
*You must beat all 6 levels with each character, so that's 24 perfect rounds in all.
*Karlsnor does not have a ranged weapon, so has the toughest time beating the Goblin Chieftain.
*You don't have to beat the levels in order. As long as you beat all 6 without dying with each character, you are fine. Any order will do.
*You can quit out and restart if you are not going to beat a boss or are going to die mid-level.
*Karlsnor depletes his energy when he rolls or uses the (Y) button. To keep his health up, use rage often against normal enemies. Be aware he's also slower while enraged.
Tips for beating the Goblin Chieftain with Karlsnor (Assuming you are level 20)
Your first few times, you will probably get slaughtered. Stay well away from the enemy and try to get him to charge you. Do not ever try to enrage Karlsnor and engage him directly toe to toe. When he charges, dodge away. Dodge dodge dodge. It depletes your energy, but it will keep you alive. You'll never have higher than 0 Energy except after a successful hit. When it's time to make a hit, jump at him, smack him in the head, then press (X+A) to activate rage and then (B) to roll away. This all takes about 1 second, so it should be fast, (X), (X+A),(B). What this does is if you are level 20, it will send a rage shock at the enemy. Since you can never use rage anyways, might as well use it for these "shocks" this is the only way i found I could hit him more than once and do decent damage. Sometimes he stays dizzy for two rounds, sometimes he comes out of it immediately, that's why it's important to predict this move and dodge away each time after 1 hit. (And the rage if you have it). So... to reiterate, I'm not saying to save your rage, I'm saying gain the little bit of rage for your first hit, use the rage immediately, and dodge away. If you try to swing at him twice, you'll end up getting hit back and he hits really really hard. This strategy should be more than enough to beat 1-3. But 1-6 is another matter
Beating two chieftains 1-6
Now that you have the, (X),(X+A),(B) strategy, you should be able to take on both of the bosses, but now the stakes are different since they can pin you down between them. My strategy here was simple, use the middle pillar. I was never once in danger of dying because I stayed on the pillar 95% of the time. The middle pillar has two structures roughly as tall as the Goblin Chieftain sticking out creating and angle of roughly 90% or so between them. Do not bother standing on these, stand on the ring around the pillar that is on the same level as these two structures. It doesn't seem like you can, but you can. You can literally run around the entire pillar without falling if you practice. You can't be hit by almost any of the Goblin's attacks if you are smart and keep the structures between you and them. Wait for the Goblins to charge, and jump down and hit, then escape between the space between the pillar and the structure, and jump back onto your ring immediately. Always use (X),(X+A) and (B) to dodge away. You can even use (X), (X+A+B) I don't think it really matters as long as you are dodging away from the enemy and not trying to hit him twice. This took me about 30 minutes, but I still had half my health and never died. The enemies leading up to them may have been the cause of some of my damage, I'm not sure. I'm almost certain you could beat them both without ever getting hit once.