Where are you in your campaign?

Specifics will depend a bit on your playstyle, but some general tips:

• Be sure to check the '?' button on skills for additional information on how/when they apply or how they apply differently than similar base-game skills. Some have some information on how they may pair well with other skills, etc.
  
• Use the control abilities if you are getting overwhelmed - Suppression is a very powerful tool and a Sapper with Flashbangs or Concussion Grenades can give you a lot of CC tools.
  
• If you are already getting shot at (that's not a bad thing, if you are alpha striking everything before it can act you haven't increased the difficulty enough IMO), then don't be afraid to take defensive skills and things like Return Fire. A lot of people shy away from defensive skills with the mindset that it's better to kill the enemy first - but sometimes you can't...

I'm currently in mag weapons now but about to upgrade to Plasma. So far no alpha strikes as the enemies are just too big in a pod because of the Hive Mod.

I find the tech specialist really underwhelming and sometimes I don't see the effect of the skills and abilities as I'm still getting adjusted to them plus which primary weapon to use on each class.