One way to get this is to create a Sorcerer character (they have the lowest HP at level 1 of the four classes) and then, rather than playing the campaign, quit out and select "quick battle" from the main menu setting the difficulty to 'hard'. Your two options will be to battle a Swarm of rats or a Goblin. Pick the Rats as they have a higher physical attack (6 verses the 3 of the goblin sword) and they tend to focus on the board more (occasionally the goblin will opt to use his sword rather that the skulls on the board which could cause a cascade of damage).
The enemy will always go first so just keep quitting out if they fail to kill you in one turn. There are a selection boards you will randomly play on, a few of these will be massive open boards and others will have 5 damage skull gems at the bottom, these are your best bet for unlocking the achievement. Personally I came closest (2,3 and 6 health left) on the board with the three tightly packed 5 damage gems in the bottom right corner and also got my achievement from this board this around the 10th or so time I came across it.
All in all it took me about 20 minutes to get the achievement using this method but it's entirely based on random chance so could take more or less depending on your luck.