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Here's an easy way to get this achievement:
First, make it to England.
Next, use the God Mode glitch shown in this video:https://m.youtube.com/watch?v=70I6g_nMrG0
(NOTE: The video is not mine.)
Finally, travel to this location and fight the Lynx, only killing the large one:
You can then walk away and it should respawn. Rinse and repeat until you feel ready to apply them to your skills, which improves your power rating. Keep killing the Lynx until you're power level 400. Then use another ability point for the achievement.
UPDATE: Looks like the Lynx was patch (but not God mode). I'm currently looking for another way to farm for XP.