The thing is, IM is not going to have a class system. An excerpt from this article:
Playing a pre-made character in a game can be great fun, but when designing Interstellar Marines it was pretty clear that we wanted more than that, we wanted to give players the freedom to shape and mold their character themselves.
In the game the characters will enter the solar system as rookies, but during the progression of the game all gradually become more and more experienced as the story unfolds. With experience comes new skills and abilities as well as access to new and better weaponry, and it is now all up to each player how they want to build and equip their Marine.
Create a stealthy medic with hacking skills, a heavy armored sniper with explosives or maybe an officer with excellent strategic abilities and a minigun?
It is all up to the player to decide.
Saying how the leveling system would work is pure speculation right now, but based on that description I would guess either a perk or a point buy system. After every mission you get some perk selections / some skill points to train. Here's the thing though - In that environment, the OBFAB would be available to everyone, and if the legged sharks are as dangerous as they should be, everyone would get it. Conversely, if you nerfed it to lower than 100% success rate, nobody would get it.
I had a similar situation in a game I had played not long ago, Dead Space. There were all kinds of things trying to chop me up into little bits, and it had some really tense moments when things got too close. That is, until I got the force gun. For those of you who don't know what I am talking about, I will elaborate. Probably 90% of the enemies in Dead Space were melee, and you couldn't do a whole lot but try to knock them back and run away at melee range. The force gun basically hammers things with a gravity blast, think the illegitimate child of the hl2 gravity gun and a shotgun. It basically took ALL tension out of fights. If things got close I just switched weapons and knocked them on their ass. Yawn.
I want to be scared of the sharks. I want to be put on heightened alert and get a little paranoid at any hint that they could be present. I want them to be deadly as hell. I don't want an "i win" button that neuters all tension and pressure I am hoping I will feel when fighting them. I don't want to be Zangief the shark wrestler. I want to control my character to do cool things, not push a button and watch an animation of them doing cool things.
All of the above statements were simply my point of view. I welcome counter-arguments.
P.S. Oh, and how much do you think a legged shark would weigh? I'm guessing in the +400kg range. I realize you can do a lot more than normally by throwing with momentum, but I think that is pushing it a little :)