The first and most important thing that you need to do is to find a suitable spot on the map to cast the Skull Trap. A suitable spot would be one where you can attract a lot of monsters at the same time so that they get affected by the Skull Trap. You should search for a spot that offers a large, empty area. You will have to spend a fair bit of time scouting the map to find such an area. To keep yourself from attracting any unwanted attention, either make yourself invisible, or simply use Farsight, depending on the type of monsters in the area.
One extremely important thing that you need to keep in mind is the radius of blast resulting from the detonation of Skull Traps. Whenever a unit, including your own character, come within 20 feet of the Skull Trap, it is triggered, resulting in a huge blast that damages all enemy or ally units within this radius of 20 feet. You need to ensure that the area where you set up the Skull Trap does not have weak monsters in it, as these monsters may end up triggering the Skull Traps and consequently waste them.
Start setting the Skull Traps. Memorise the Skull Trap spell by using all your third-level spells and then cast them again. Continue to repeat this process for as long as you have casted enough Skull Traps to take down the last enemy in the encounter area.
Since you cannot spam Skull Traps freely, it is important to use them in such a way that they affect the maximum number of enemies. Therefore, after you set the Skull Traps, carefully go around them and catch the monsters’ attention. As they start chasing you, bring them to the area where you have planted the Skull Traps. The job of luring them to the area will become even easier if you use Haste or Boots of Speed.