Make sure you have purchased the Ranger character DLC, then start a new local game with the Ranger as your character of choice. Play against whatever you want, as long as you're confident you'll survive. (Making the opponent a local player you control makes this easier, of course.)
Once you start the game, use any runestones you have that increase your starting Strength score and lowers trade in cost for Strength trophies. Then just play the game as normal, focusing on increasing your Strength score and fighting Battles against enemies.
The achievement should unlock in no time at all, after a fight with an enemy with a lower Strength score than yours. Probably about 10-20 minutes, but that's all dependant on your luck with the cards. If this takes you longer than 30 minutes, you're probably really unlucky with the cards drawn or the dice rolls.
Note: I believe it's the total difference in the combat that counts, not just the character's Strength. So weapons, spells, and magic items should also help with this.