Let's not try and chew into lore TOO much, because if you really want the truth about Space Marines is a squad of 10 could in fact take down an army of 1000 guardsmen alone. They say it takes 100 normal men to match 1 pace marine in effect, so we can't really go that realistic. Also, Space Marines have one of the fastest regenerative abilities in 40K, flesh wounds heal nearly instantly, deep wounds heal in minutes, and they can suffer the loss of several key organs and still survive. (Namely the loss of one of their hearts, one or two of their lungs since they have 3, kidneys; all 3 of them, their stomachs, 3 of them, and their secondary liver) Orks on the other hand can be killed more easily, but their flesh is indeed tougher than an average humans' and their bones far denser as well as those Orks killed can be "salvaged". Their limbs can be removed and reattached to other Orks so long as the nerves in the limb are intact and Orks can even survive a head transplant though extremely rare. Now, in terms of the game itself, a large health pool is a must, but it would still probably be lower than Space Marines as well as their armor and resilience. Their true shining moment, aside from hopefully large numbers, can be their burst damage. Orks alone, when enraged, are frighteningly strong. Orks in a mob with the psychic resonance of the WAAAGH! are truly ferocious and there should be a mechanic that multiplies all damage by a flat amount depending on how many Orks are nearby. THAT is my solution Behavior! Please consider!