Why would you use it while you are surrounded? His ultimate is the most damaging and accurate ranged attack in the game. It is not a single shot that you can miss; it’s a constant stream of damage that will respond to any aim adjustments you make the instant you make it.
Cooperate with your teammates! Did Reyna just give you an overshield? Use it. Is Montana distracting the enemy with his mansformation? Start melting. Is the enemy being slowed on Thorn’s blight? Make them feel even more pain.
Create your own setup by deploying your shield, hit the enemy with a slow mine, and then activate core discharge. Oh, and stop aiming for their head… Hitting them consistently in the legs or the torso will give you better results than flailing the beam around at a tiny target.
Even if you do not kill anyone, you will force them to retreat, which gives your team the advantage. You can also kill a giant minion or thrall with ease, not to mention clearing a minion wave when it’s urgent and you haven’t leveled up to 7 yet.