When you finish rounds 2, 5 and 7 you get checkpoints, which makes it much easier. Even on Easy, Frost has incredibly strong punches. So no matter what the situation, try to stay away and standing up until the 8th round.
Stay up! You're going to need to be able to get up in later rounds. Frost's punches are incredibly powerful and he can knock you down in seconds.
You have to land 75 body shots in three rounds. You can either get them out of the way in round 3, or you can take your time, watch your health/stamina meters and spread the 75 punches throughout the 3 rounds. Stay low and back off when you need to. Stay low, clinch, move around, generally stay away from Frost. Either way, it's still imperative that you stay up.
A cut will form over your eye and you have to protect it. Keep covered and keep moving. Stay out of Frost's range as much as you can. Keep trying to stay up.
This is your time to take Frost out. Your cut has stopped bleeding, Frost is tired and you get a great speech from Gus. Go after Frost just like any other fighter. You should be able to knock him out in the 8th. The achievement will unlock when Frost hits the canvas.