Bad North: Jotunn Edition

At first this might want to look like an achievement you want to get at the end of a campaign. After having about 8 or more commanders. But I can ensure you, that this is definetly not the case. At the end of every campaign the combats will be really hard and you will need 4 commanders in every fight.
So the strategy for this achievement is to start a new campaign and try it right at the beginning. Fight at each island to collect enough coins. For your both initial commanders select the class Infantry. After visiting the first island with a new commander you should upgrade this commander as well with the class Infantry. Now you can try to get this achievement. Send out one commander to an island, then (without pressing 'X' to end the round) send the next commander to another island and afterwards the third commander to a third island. Your commanders with the Infantry-class should be strong enough to kill all vikings. Especially because in the beginning of a campaign there are only weak enemies and not much of them. After winning the three islands in a single turn the achievement will unlock.

Hint: The difficulty of the campaign doesn't matter so you should definetly start a campaign on normal and not on hard. And if one of your commanders is nearly defeated while fighting alone, send him to one of the houses to restore the troop. For this achievement it doesn't matter if you lose one building or not as long as you win the island.