Army of Two: The 40th Day

I found an easy way to do this:

Things you will need:
-$18,100
-RPG7 available for purchase
-Fleur-de-Lys style for purchase

Step 1: Purchase the RPG7 ($18,000) (The Aggro should be maxed out)
Step 2: Purchase the "Fleur-de-Lys" Style ($100)

Achievement unlocks.

Note: I bought the "Pimp" style because I know it increases Aggro from the previous game. I believe the achievement pops because the Aggro is already maxed and increasing it satisfies the achievement condition. If this is the case, then any style that INCREASES Aggro should work. Since the RPG7's Aggro is already maxed out, you won't be able to tell if Aggro is increased until after you purchase it. If anyone finds out which styles increases and which styles decreases Aggro, please comment and I can add into this solution.

To customise a weapon, and when out of combat, hold down 'Y' and press d-pad down. (if playing co-op, your ally will have to walk over to you and press 'A' to accept) Then, chose the weapon you want to have high aggro on, highlight it by selecting it with the analogue stick then press 'Y'. Proceed in modifiying your weapon untill you get the aggro stat maxed on your gun. A quick way to do so would be to equip a frontal shield and the bling skin to your gun, combined with any other aggro raising weapon parts...

It is worth noting that you do not have to have your aggro completely maxed out to get this achievement, it just needs to be "high".

I got this achievement by taking the 870 shotgun, adding on a new barrel, the homebrew shield and the shotgun bayonet. Then I added on the "pimp" style, just to be on the safe side. My aggro for this weapon was almost maxed out but not quite and the achievement still popped