Currently it seems that aliens have an advantage at stage 1, marines have advantage at stage 2, and prehapes aliens have a slight advantage at stage 3. Allow me to explain:
At stage one dragoons are availible, these units are capable of getting headshots on marines with a fair amount of ease which will one shot a full health marine without a helmet. This allows a good dragoon to jump into a pack of humans and take down 3 or 4 within seconds. Marines have no item to counter this at stage one, and their guns lack the power to gun down a dragoon before he gets in range for that one single bite. Stage one deciding force: Dragoons
At stage two marines get the helmet, which gives 70% damage reduction to the head. This makes them extremely resliant against most aliens as a dragoon must now score 3 headshots to kill a marine. Also unlocks the pulse rifle which is rather effective against the large targets. Aliens get adv marauder but the zap really only does good damage to marines who are unarmoured. As a marine with light armour will still take 4 direct zaps to kill. Stage two deciding force: Marine armour
At stage three the tyrant is unlocked. While not incredibly hard to kill this alien seems to be at the perfect height to always get headshots without much effort. It also hits very hard. This allows it to drop most marines extremely quickly including battlesuits in 3 hits. Marines get the battlesuit, which ups reduction to 80%. Also unlocks the lucifer cannon, which while great against smaller aliens due to splash, lacks punch against hit and run tyrants. Stage 3 Deciding Force: Tyrants
Of course balance "complaints" would be unsavory without suggestions as to a solution.
The solution I offer is rather simple, change the damage model to the .5 to legs 1.0 to chest and 1.5 to head model that some other people have offered. This would prevent one shot headshots early game cause by dragoons and would tone down the tyrant slightly without nerfing it's damage to buildings. Damage on dretch basi marauder and prehapes even dragoon could be upped slightly to make up for this small loss in overall damage.
For stage two I suggest that light armour + helmet reduction be reduce abit, prehaps down to around 50-60%. This would prevent marines from suddenly gaining "tripple" damage absorbtion from a mear 160 credits at stage two.
Feel free to comment, offer suggestions, or "object" but please try and keep it constructive and offer something meaningful to the thread. Thanks.