Steven blitzs and oneshot lol man only around 3 characters in the MHA verse are around mountain level meanwhile base garnet was that level early series, steven also is much faster and has superior reaction time
Steven's best chance is if he uses his scream at full power which is enough to send a powerful shockwave through the Earth's mantel at minimum, well beyond anything ever showcased in MHA.
the problem is that Steven has only ever showcased that level of power as monster Steven not to mention in character Steven won't be going for the kill at first.
Steven should have a solid strength and speed advantage over Shigaraki due to scaling to other characters like Garnet but Shigaraki has much better regeneration and it will only take one direct touch to kill Steven.
Steven probably loses because his trauma won't let him kill Shigaraki after killing Jasper and nearly killing White Diamond and Steven can only turtle up and trap Shigaraki in barriers for so long, but if we remove Stevens morals Steven would probably win.