Being level 60 doesn't make me a super hard bad-ass, but having a level 60 Deathstalker Zed's Demise sure does
Thanks for the tips invincible. I've reached level 60 yesterday. The first time I completed the game I was already at level 48, having done most of the side-quests, and having played several hours of bloodbath arena. I started a new game, pretty much ignoring most sidequests, and by the time I reached the lab, I was level 53. Then I spent two hours farming money in the lab, buying oleander, and trading them for XP. Pretty lame, I know, but you need over 4 million XP to get from level 53 to 60, so it's by far the quickest way.
Every time I reached a new level, I saved and quit, and reloaded my game. I did this because every time you reach a higher level, you get more XP for the same quests, if you reload the file. At level 53 the Oleander sidequest was giving me 3.094 XP each time. At level 58, I was getting over 5.000. When I reached level 59 I just went over to Svetlana, sold my champaign bottles (40 of them) for 17.000 XP a piece, and it was enough to reach level 60.
After reaching level 60 I just continued with the game until the Banoi Butcher sidequest, which earned me a level 60 Zed's Demise. Combined with Xian (especially with her useful skills concerning modded weapons and sharp weapons), and the Deathstalker mod, that thing is a beast. 4498 damage, very high durability, lethal poison,... Totally worth it.