Use your first charge to blow up the Charr Supply Carts (3), marked as "Charr Supply Caravan" on the mission map. Simply drop it inbetween the three carts.You find your first charge near the first group of Charr you defeat.
You can pick up more charges by killing Charr Firereigners or picking them up from the Charr Ammo Cart. Drop them near Charr Watchtowers to destroy them, or deploy them in a Charr Siege Devourer.
- These explosives seem to be a miniaturized scale of the "meteor" spell the charr used in Pre-Searing Ascalon to destroy the wall.