You main problem is that you are not using a fast (firing at or below 11 AP per shot) assault rifle (AR). You can see a list of fast AR
here, AP cost table.
I'm assuming you are wearing metal armor (STR 8 requirement) with 8 STR AR build, and using high caliber AR which fire at 12AP+ per shot. When you are crippled you no longer meet the STR requirement of your metal armor so you suffer a 15AP penalty (you now get 35 AP per turn). You can still burst with a fast AR (11AP * 300% = 33AP), not so w/ higher caliber AR. Remember AR have a strength requirement; Precision is decreased by roughly 10% for each point of strength below the requirement. Fast AR has a strength requirement of 6 so accuracy is not an issue in the initial round.
As mentioned in the above post you can use adrenaline shot (+2 STR +20AP for 3 turns). Another option would be a taser (you don't care about the damage as you only need the stun, so use a low quality electroshock generator).
If you can't craft a fast AR at this time you can use the following strategy: taser your enemy, this will reduce their evasion to zero (very important as evasion reduce AOE damage). Since you have 10 CON and wearing metal armor, toss the highest level frag grenade you have at your feet (optional: and scream "Self destruct!!! It's super effective!") - this should kill them outright or severely damage them.
Flashbang to incapacitates any remaining enemies and finish them off next turn.
Your secondary problem is that you need a better metal armor set with higher mechanical damage threshold. If you have a high enough mechanical damage threshold you won't get cripple by a zero damage strike. Early game you should hunt for high quality steel/tungsten plates and craft high mechanical DT armor set. You can take a look at
this table and get a rough idea on what to expect for a quad plated metal armor set. You should also use a double low freq shield as this will reduce melee damage. If you are lucky, the damage reduction might be enough to bump the incoming damage below mechanical damage threshold.
Funny thing is that there was a similar discussion awhile ago
here.
Edit: forgot to mention - a slow and heavy hitting AR is not useless. Similar to SMG, you can wield a fast AR on one hand and a slow, heavy hitting (9mm/Steyr-Auch) AR on the other and fire the slow AR w/ the free burst from Commando.