Pretty simple achievement. The characters that can use a magic shield are Horus and Isis. Any character can lay a bomb.
Just have the second player pick one of the shield characters, and then hold down
while you use
to lay a bomb and press
again to detonate it. Achievement will pop in a level or in the hub. You do not need to kill the other player, or even destroy the shield, you just need to set off a bomb within reach of the shield.
If you need any tips, or have anything to add please comment and I will get back you ASAP.