Hmm i'm not sure which part you mean. The only thing i suggest is to beat the big boss dinos you'll need to stay away from them and shoot them from a distance. Always be wary of their attacks but just keep shooting it until it falls onto its side. You will then most likely need to run up to it, draw your knife and complete an auto-kill..

I hope this is of some help...
If your still stuck try posting as much information as to where you are as possible.