The Indominus rex attack the guards after they fired at the Indominus rex. So the guards are killed and eaten by the Indominus rex while many more guards were wounded by the Indominus rex.
“MORE GUARDS APPEAR”; GUARDS! OPEN FIRE AND GIVE EVERYTHING YOU GOT! “THE GUARDS OPEN FIRE AT THE INDOMINUS REX“; “THE INDOMINUS REX KILLS MOST OF THE GUARDS AND WOUNDED MANY MORE BEFORE LEAVING”; WHAT?! IMPOSSIBLE?! WHAT THE HECK IS THAT THING?! HOW IS IT ABLE TO KILL MOST OF MY GUARDS AND SURVIVE AN ONSLAUGHT OF GUNSHOTS?!