You can get critical hits on enemies by hitting them really hard when they are either mid-swing with a powerful attack (red attack) or sometimes when they are knocked off balance (green) after you deflect. Obviously the best way to get this is to charge up you Chi attack ( + ) and hit them at their weakest moment. You'll know you got a crit if the screen turns red white and black. You have to get 50 overall and you should be able to get this by, or soon after you complete the campaign.
A great level to get a lot of critical hits on is level 5 where the sumo guys jump into the air to do ground slam attacks while chi refill items constantly respawn in the middle of the map. Hit them with your Chi attack while they're in mid air for a very easy crit. You can very easily crit everyone in the level, for a quick 20+ crits! The achievement will pop as you exit the level and reach the stats screen. You can repeat this in either coop OR singleplayer to get the achievement quickly, as the description roughly points out, your total crits does not crossover between single and multiplayer.