To beat copperhead use the quickfire on the explosive gel over and over:

It will easily eliminate the fake copies of copperhead. Once you have eliminated all of them hit copperhead as many times as possible. Shortly after more copies will spawn. Again use the quickfire of the explosive gel.

Later on copperhead will dash through the lit up area. Either keep using the explosive gel to get out of her way or dodge her. Eventually the antidote will drop from above and you are done with Copperhead.