Surely just stack ults for a full buy round. If you have 8 rounds to give sureeely you just make sure to use like 3+ ults on a retake/exec/post plant and win right?
C9 eventually won but their ult economy was kinda bad (way better than NRG 12-5)
I do this every time in ranked when my team is slipping a huge lead and I never throw games like that