Gunzerker Level: 21-25

Yippie Ki Yay: Increases the duration of dual-wielding every time an enemy is killed while Gunzerking.
Required Level: 20
Upgrade: 3 or more if needed

Last Longer: Increases the Gunzerking duration.
Required Level: 10
Skill Upgrade: 5

I'm Ready Already: Cooldown rate.
Required Level: 5
Skill Upgrade: 5

As long as you have Last Longer and Yippie Ki Yay. Shouldn't really matter what else you pick from the tree.

Have your friend rank you up to Level 28 by them killing either Terramorphous, Pyro Pete, Pyro Pete's Bar Minons, The Warrior or Rakks from the Warrior over and over on there host from there Lv. 50 True Vault Hunter mode playthrough. This can be done any way to get easy XP.

I would say it took less time to rank up than playing the whole story over if you wanna play the whole story that's fine. Either way you should get the achievement around Lv. 21-25. The reason I went for 28 was because there was no guns in the game I could use at that level. I had to go to T.K. Baha's House in the Badlands for guns while I played on my friends host.

I used two shotguns for this achievement. I bought shotgun ammo backpack upgrades to do so. Any guns will work even grenades.
If you follow or view Randy Pitchford's (President of Gearbox) or Gearbox Twitter/Facebook (You don't need any accounts) or the forums of Gearbox they give out shift gold key codes to use on a chest in sanctuary. You may or may not get actually weapons but it beats looking for them.