From Halopedia, the Halo wiki
Only the first and the second Rhino are at risk from later, Covenant attacks.
Another way is to place two fully upgraded Wolverines next to the Rhinos.
Rhinos must be at full health before the 5th and final Rhino is placed at the final destination or else the achievement will not be unlocked.
The simplest approach is to simply let some of the Rhinos take damage and prior to placing the 5th Rhino on it's position, make sure that you cast the Spirit of Fire's regeneration ability on all Rhinos to bring their health back to full. As long as no Rhinos are destroyed and they are at full health before the 5th Rhino is placed, the achievement will unlock.