Its obvious. Pro Aliens will simply CRUSH humans even if they are camping. And SD makes it even worse in some cases.
In all affairs to increasing human resistance? People want new things like buildings, and weapons, heck even accessories that are new besides battery or jet packs.
I say we should have better Ammunition.
For instance,
When you have a Rifle, you can buy the free standard ammo, or the little costly 'Advanced Velocity Ammo' for x amount of credits. It would simply fire faster, do more damage, and just be more helpful against Goons or Marauders at s1.
And so, Weapons could be like:
Shotgun: Increased Bullet Capacity. Increases amount of rounds fired in each shot.
Lasgun: Over-Charged Battery Clips. Increased damage. Faster Fire Rate.
Chaingun: same thing, Advanced Velocity Ammo.
Pulse Rifle: Over-Charged Battery Clips, or dare I say it, 'Homing Shots'.
Flamer: Chemical Gas. Increased damage and range. Turns the fire blue or green. (Or Purple

)
Lucifer Cannon: Homing Shots, Over Charged Battery Clips. (Does damage, not firing rate.) And a free ammunition, Increased Velocity Shots, at the cost of lower damage.
It came out of my idea after reading the Alien Armor post, but I have suspicions that this will turn out to make Humans TOO strong.
But it does give humans an advantage in SD.
But It DOES cost you.
For some guns how ever, it could be that you can simply buy it once, then just go back to refill your stock.