The Plague Marine rises out of cover, the plasma cannon in his left hand. With his right hand, he points directly at the sniper. With that, he brings his plasma cannon, setting it to maximum power. He then lobs a burst of plasma right at the unfortunate sniper.
Spoiler
Aim, then single shot (maximal) Target 61
(1d100)[15]
Damage
(3d10+15)[30] Pen 12