Originally posted by Nascarman:

the arbalest is the one true counter to gjalls, if you can get to an area with visibility and some distance you can stun lock them with the crossbow knockback and they will barely be able to fight back, often only getting a single attack in right before they die

this+ hit the big red underbelly & the crossbow will drop it in 4 shots. Any sort of viking tall terrain will block the firebomb.