This game rocks but the only thing that i have some problems about this is that if your level ends with an even number does have a skill point but if your level ends with an odd number, it doesn't have a skill point. Please increase a skill point when leveled up.
The real reason why alien campaign is too hard is this: 1. they're not making a cease fire when an enemy is having a static shield. 2. they want to be alone so you need to follow them and 3. they suck because they're aggressive when they attack their enemies and sometimes aliens still following their enemies and they want to kill them. Click + if i'm right.