[FailerZ]

PHP Code:

public OnPlayerWeaponShot(playerid, weaponid, hittype, hitid, Float:fX, Float:fY, Float:fZ)
{
    if(hittype == BULLET_HIT_TYPE_PLAYER)
    {
        if(hitid != INVALID_PLAYER_ID)
        {
            new Float:hp;
            GetPlayerHealth(hitid, hp);
            if(hp <= 5.0 && GetPlayerWeapon(playerid) == WEAPON_DEAGLE) return 0; //Return 0 means the one who got shot (hitid) is not going to take damage
            //Change the hp as you wish it but that's how i understand what you said
        }
    }
    return 1;
}