What is the utility of Onslaught and Upheaval?

[Jekyll]

Shouldn't Onslaught be changed as to not use gauge and Upheaval be changed to actually be useful?.

Can't tell if you're trolling about upheaval or not. Fell Cleave is 590 potency for a cost of 50 gauge, which is 11.8 potency per gauge point (590/50). Upheaval is 450 potency for a cost of 20 gauge, which is 22.5 potency per gauge point (450/20). In other words, upheaval gives you more damage per gauge.

Use upheaval on cooldown. Onslaught is situational. I recommend https://thebalanceffxiv.com/ and it's associated discord for any further questions.