This one's going to take a lot of grinding. The last character upgrade you need requires you to be at level 49 for it to unlock, which is going to take a while (it says level 50, but I guess it's glitched because the upgrades seem to unlock a level before the description says). The best way I've found to do this was to keep playing the last "Defeat Bane" mission over and over again. It takes about 4-5 minutes to beat and nets you 1,000 XP and 100 coins each playthrough. The best part is, when you beat it you automatically respawn at the beginning of the mission again, so you don't even have to go back to the main menu. The achievement unlocks when you exit the menu and go back to the main game. If anyone finds any quicker solutions don't hesitate to comment, I'll edit this post with any additional info. Good luck!
EDIT: To clarify, you ONLY need the upgrades in the "Character" section of the Tech Shop.
J Schwizzle has provided a rough time estimate for how long the grind will take in the comments. Here are the approximate level-up times from level 30.
-- Level 30 --> 3500 XP per level --> ~20 min/level
-- Level 35 --> 4500 XP per level --> ~30 min/level
-- Level 40 --> 5500 XP per level --> ~35 min/level
-- Level 45 --> 7500 XP per level --> ~45 min/level