Primary attribute Agility

Stat spread

str base 20 +X
agi base 20+ X
int base 5+ x

Ability 1, Basic training - the marine has been through vigorous training to make the requirements needed to be on the job. He gain 10% evasion and 10% critical strike (5X) to scale adds 2% evasion per level and 1X crit per level up to 10 levels.

Ability 2, Specialization - the marine has a numourous amount of specific training and can only harness 1 at a time. You an choose 1 of the specializations to help the marine in combat.
special 1 Grenadier - the marine uses explosive rounds causeing his attacks to have a 250 range splash
Special 2 Advanced marksmen - The marine uses his adept ability to aim and gaines a 15% attack bonus
Special 3 Under pressure - the marine gains an adrenaline burst causing him to gaine 50% attack speed an 15% movement speed
Special 4 Sniper - The marine takes defensive stance and looses 20% movement speed but gaines X amount of range. The ability also gives him extra sight range

Ability 3 - Surival instincts - the marine has the ability to regenerate his health over time as long as he is out of combat by on the scene first aid

Ultimate ability - Upgrade - The marine is able to find a set of tools and wih his engineering backround is able to upgrade his armor and weapon. The process takes time (far lvl seperation) but he slowly becomes the perfect marine and has the ultimate suit.

Increase base stats + attack by a ratio dark chaos deems fit

I feel for this character he could start at he basic space marine suit but as he upgrades te skin of his suit gets different to suit the different level of upgrades

--------------------------
EDIT

Ability 3 Nade - the marine uses his training to throw in a grenade dealing 5X agi. in a 300 aoe area