I'm a Lv16 Commando Full STR build to date. Just like share some of my opinion about Commando's skills.
I will only talk about the Skills available to my level.

So far, I found the best skills is Singularity that target a single enemy, after cast time the enemy will blast
an AoE damage & knock back around him. Also this skills works for enemies, included human & aliens.

Below are some of my experience in different maps:

DYNASTAR LABS
All enemies are human, so before you have Singularity, Neutron Stomp play a big part here especially your
level is still low. Keep knocking down your enemies around you, it increase the chance of your survival. This is
extremely important especially when you are doing it alone.

REDSUN PIRATES
Fairly similar to the previous campaign, enemies are mostly human except a few T-Rex (i like called it that).
You should have Singularity now, so take turns on using it with Neutron Stomp while you are waiting for the
cool down. With this the enemies can hardly ganged & overpower you.

CYCORP INDUSTRIES
This campaign is fairly tricky especially after the second mission. Enemies are mostly alien bugs, they are fast
& deadly. I found that Neutron Stomp's knockdown not effective here (correct me if i m wrong), so the best bet is
start the first hit with Singularity, keeping shooting and running in circle to evade while waiting for the cool down.

DELTA 7
I'm still leveling hard in here. All enemies are alien bugs, plus those human controlled by Head-sucking bug. They
are tough, fast, and will explode. My combo is Singularity + Graviton Slam, which can damage & slow down the enemies,
also need Increase Mass for armor buff.

If you have more to share kindly post it up. Thanks~and happy playing!