Replying to Confronting a body shop about shitty work?

Alright my car was backed into about 1 year ago. It was hit on the rear drivers side door/quarter panel. It caused about $2,000 worth of damage to my car and the other persons insurance paid for everything. I was given the option of going through the insurances body shops or picking my own. After searching in my area I found a shop that had good reviews and everything. So I got the body work done at the shop I chose. They fixed it quickly and did a good job (or so I thought). About 2 days after picking up my car I noticed a spot where the paint had chipped off around the bumper. So I went back and asked why they hadn't fixed that spot. The owner of the shop took my complaint as insult and start saying that was there before and definitely wasn't happy I brought up the issue. I just gave up trying to argue and was fine living with a small spot of missing clear coat.

Fast forward to now, over a year later. I notice I have a large crack where the body work was performed. They used bondo and did not to a great job. I took my car to another shop to get a second opinion. They told me the body shop that fixed it used too thick of a body filler which caused it to crack. So now I'm wondering how do I confront the owner of the bodyshop without sounding like a dick but also making them fix it? It's obviously going to need to be refilled in with body filler and painted so it won't be cheap. They say the have a warranty up to 3 years later but I'm sure they'll try any possible way to get out of the situation. I'm also 19 so no one ever takes me seriously.

Sparknotes- Had bodywork done to car over a year ago. They did a shitty job and now it is fucked. How do I get them to fix it without coming off as an asshole and threatening them?