If you prefer to pick enemies one by one, sniper rifle is not so good, because it is loud. You will kill one enemy, and everyone esle will be alerted. It will be easier to do with crossbows, melee or psi. About skills - usually you don't need to raise stealth, lockpicking and hacking,more than 40-50. If you prefer to be stealthy, you need to raise them to these levels, and then you can switch to other skills, like crafting. Bear traps is a potent option even with 0 traps skills, so every character can use them. Also caltrops and some grenades are useful even if you are not a dedicated throwing specialist. Key to succes is preparing for batlle and thinking outside of box. You will win most battles mostly because of clever usage of terrain and tactics.