I'm on my second playthrough and I've completed all side missions and collected all dead drops/blast shards. I'm in "Mission 40 Aftermath" after the ending. I still need like 20,000 XP to get unlock all of the powers (good and evil) for the last achievement.
Running around and killing random enemies takes a while, and UGC missions seem to give very little XP. I'm evil, so killing hoards of civilians gives me a trickle of XP too, but it doesn't seem very worthwhile.
How can I grind more XP after the end of the game so I can get the "With Great Power Comes Greater Power" achievement?