I found it cost/effectiveness is best to use rexs after they are tamed put them in a pen with out the trodon whistle or set it to neutral then knock it out again (has to be your tame and has to be on neutral and is best to do it at night seeing how they get 2* the tame bar then during the day (when there eyes are glowing) the higher the rex the better or have a few on stan bye lower there health as low as possible to so the trodon will kill them quickly aim to get two trodon's of opposing genders then just breed them past that -add on Xbox HARDCORE PANDA7
At lvl 72 with flak armor on, a lvl 5 Troodon knocked me out with one attack at night. Last thing I saw was those glowing eyes before passing out. My Guess is that they attack vital organs. Luckily my Doedicarus was up for a game of baseball and launched that little devil out into the stands.