An easy way to score the x50 combo needed to earn this Achievement is to reach Round 4 of the "Intensive Training" combat challenge. At this point, abuse the trick of attacking enemies, then jumping over them to avoid their hits/counterattacks. Simply continue this until you reach a x50 combo, without worrying about using combo attacks or finishers (indeed, these attacks will finish enemies off quicker, potentially throwing off your timing). It gets even easier if you have unlock the Shock Gloves, which provide a x2 multiplier on each successful hit when activated.