My ds has two bottom teeth as well. He usually only bites right at the end of a nursing session when he is essentially done and wants to play! But last weekend he had a cold and couldn't breathe through his nose... poor baby. Well, he would get very frustrated and bite me. Bad enough that it made me cry. But, it was such a hard spot cause I knew why he was frustrated and I just wanted him to eat if he could. Needless to say we made it through and now he's back to not biting as long as I pay attention and don't let him "linger" too long. Also, if I can catch him when he is sleepy it really helps.
FWIW I tried many things that people suggested, setting him down, telling him no bite, setting him on the floor, smooshing his face into my breast, etc., etc. What I found is since he was doing it at the end he wasn't really bothered by these things and actually giggled when I pushed his face into my breast. I think it really depends on the baby and how sensitive they are as well. Good luck cause I know how much it can hurt.