This is a fairly simple achievement, All you have to do is win a game using only bazookas and grenades, So what you want to do is from the main menu is
1: Choose Local game
2: Go to versus and choose deathmatch NOTE- where its says insert team put CPU into the first slot and after you have done that click on CPU and put to easy and put the number of enemy worms to one (two if you want)
Next Insert your own team in the next slot and then start the game
Once in the game you can use as many grenades and bazookas as you prefer
So for example you could bazooka the enemy on your first turn and then use a grenade next turn and then finish them off with the bazooka.
Just make sure you damage the enemy at least once with each of these weapons and at the end of the game you will get the achievement.
This only took me about 4-5minutes against 2 enemy worms.