pre ts Luffy was able to almost save Ace, he only didnt because Ace was a complete idiot.
Mihawk taking part in it or not, even if it was just post TS Luffy alone, he would have been able to save Ace. Remenber, WB entire crew would be still there, and Luffy and Zoro can do well against yonkos. Luffy wouldnt be almost killedd by Akainu in this state and Ace wouldnt had to shield him.