That's my problem with systems, Satoshi, I've always wondered how I would go about doing something.

I think every character should have Vitality, Defense, and Aim.  Equipment like Armor and Weapons would increase these attributes, and surviving characters would be able to 'grow', adding permanent bonuses to either of the three attributes.

[i]Before [/i]a battle I would give specific descriptions of the area (and further descriptions can be explained via OOC, if asked).  Are you on high ground or low ground?  What's ahead of the group, to the left, to the right, behind them?  What's in the area?  Trees, brush, cars, buildings, etc?  'Full cover' increases Defense more than Partial Cover, and No Cover confers no bonuses at all.  Each player needs to explain in their post where they're going and what they're trying to do.  I'll also provide details as to how far away certain pieces of cover are and such.

During a battle, I want to keep a nod to the 'turn-based' gameplay present in the games.  So everyone posts once, explaining their actions.  Then, I post once, detailing each action and its consequences, and explaining the alien's/EXALT's actions.

Let us say you want to shoot an alien.  First, take your Aim.  Let's say it is 6.  Now roll a d20; you get an 8.  8+2=10.  Now the alien/EXALT rolls a d20 and adds it to their defense (equals 6).  Since 10 is greater than 6, your shot connects.  You look at your gun's damage value and the alien/EXALT's Vitality is reduced by that amount.