Don't waste your ult on 1 enemy. Except when it definitly can save your life. It has a delay, so don't use it on low HP, coz you will die, while it's activating. Some champions can easily defend it, like Ghrok, Zhin. Shields also can counter it. Some others can counter ult, like Lian, Grover, Fernando. When you use your ult be sure, that they have their counter skills and ults on cooldown (use your ult just after the counter ended). It's better to not ult on a whole match than ult at the bad time, and later you miss a good moment. Use ult, when you have max ammo, and you have Rend Soul. Start to shot your ult right after you've thrown it, coz your shots need time to reach it. When the enemies will pulled in, your shots just reach them for the max damage output. Use Rend Soul asap after you collected the max (4) soul charges on the enemies. Your shots goes through the enemies and have huge hitboxes, so don't hit fully an enemy, rather shot between them for the multiply hits at the same time with the same orbs. If your enemies are on the left and the right, so you couldn't shot both with any orbs change angle after thrown it away, and stand in a line with them. After Rend Soul you can chase them down if they still could escape somehow. So go closer to them while you are ulting for being able to chase. Your ult is better, if there are an ally, who can shot the grabbed enemies too. If you have a Jenos mate make ult combos. If your Terminus just starting to Reanimate ult onto his body a little bit closer to the enmies, if there are any. If you have to ult on low HP than use Rend Soul a little bit earlier, for selfhealing before they can effect Cauterize onto you.
Mostly just useing it after their F skills and ult went out and put it between stacked enemies will help you so much. Never ult super immune champs, Like Koga, Evie or Zhin. They just can counter it whenever. And don't waste it only on tanks. If their tank is shield spammer timing your ult to right after the shield went down, or the tank will counter it.
After your ult not just chase, but use 'attack' vgs, and push them back to their spawn. You should push them too from safe positions where you are far enaugh, but you still have good sight onto their base. It's a little bit similar to the sniper spots.