Choose a Sniper as your main char.
Always you will need a Sniper in your party and also he is the faster start, you will be soloing missions, without taking damage and one-hit kill everything, always getting the missions achievements.
Save up your free gold bars (real money of the game).
You will need it to buy a permanent merc, I suggest a Gunner to convert him in a tank, and your Sniper will be the DD of the party (Damage Dealer).
: skill the Gunner only on strength and endurance.
Use wisely the skills points.
Every class “max” his weapon skill at 60-70, this will permit to use the best weapon for their class, so I suggest 70-100 skill points on Sniper Rifle for the Sniper and 70 skill points on the Machine Gun for the Gunner.
You can potentially distribute your 150 points (3 points per level over 50 levels) in such a way that your merc can use 3 weapons reasonably well. Max the damage dealing weapon and then a backup weapon that uses low AP.
For the body skill points I dont know how to spend it in a sniper, just focus on damage dealing and 0 protection.