I'm thinking that if Mecha Frieza had fought with all his power, maybe Future Trunks SSJ before splitting him in 2, would have given him a strong blow to the stomach and then split him in 2 with his sword. Or maybe Future Trunks SSJ would have similarly killed him, but it would take an instant longer to slash him in 2 (maybe something similar to when he slashed Gigant Zamasu in 2).
Either way, Future Trunks SSJ would have finished Mecha Frieza in an instant, I like to have them with a gap of 1,333..., similar to Vegeta's advantage over Cui, strong enough to finish him off very easily and very quickly.
I think Mecha Freeza used his 50%, but when he launched his Ball of Death, that attack had access to his 100% and as we saw, it didn't hurt Future Trunks SSJ in the slightest.