This trophy is for killing the ratoids (the metal enemy rats) with the red explosive TNT blocks. This needs to be a result of the blocks exploding and not just dropping the blocks on the ratoid to kill it.
If you don't get it naturally or want to get it done quickly then use level 38. There is only one ratoid but it is quick to set up. Use either of the explosive blocks at the bottom where the ratoid is. When using the straight line block use it in the horizontal position and for the l shaped block use it so the three blocks are horizontal.
Use the square button on your controller to speed things up and when the ratoid explodes you can restart the level. There's no need to complete the level either so this can be done very quickly.