When you first fight the warrior, if you die, you will respawn at the begining of the area. As you run back to where he is, there is a giant shield that you drop down under.

Instead, get up on the rocks on the left. You can snipe his head if he is straight ahead or to the right, but when he comes to the left, you get a great view of him and can unload your rocket launcher into him. Run back to the machine for more ammo which is likely needed.

His health will drop to zero, then he will just chill until you run down there and do him any damage. Then Jack will start to whine and all that.

If you return to farm him for goodies, there will be no shield, but you must go down to get him to load. If you die, you will be back up top, but now there will be no shield. Just stay up top and kill him without fear of getting hurt.

So you basically sacrifice one life for an easy warrior kill.

Bonus: Bank3r: If you use Zer0's BORE skill, Bank3r often dies in one shot.

Otherwise, stay below where the vending machines are and shoot at him through the space between levels. It is almost impossible to get hurt by him there, unless you try to get hit. Use the machines for ammo, never listen to Brick's advice...