Hey, not a solution but some tips that helped me getting the achievement (I got on WotC):
-Make sure your main squad goes on several missions from the Resistance Ring. This cover OPs rewards soldiers with bonus points either on ACC, MOV, WILL, HACK, etc. This will compensate for carrying lesser quality equipment.
-Make sure your main squad has a many abilities from their available pool. Passive ones are the very best, including: holo targeting, shredder, volatile mix. Hopefully you'll have your holo targeting soldier taking the first shot, then the shredder will go in second place, so anyone after them will have improved aim and will have to deal with less armor.
-Make sure you bring only superior quality upgrades for your weapons: give your sharpshooter the biggest ammo capacity so he can use Kill Zone at any time. Soldiers with the Hail of Bullets ability should have a larger ammo capacity upgrade too. Finally, bring one or two Repeaters... those things procs more often than you can think... my commando just excecuted one the final bosses on the second shot.
-Last but no least: psi abilities are 100% aim most of the time and give S**t about armor, so psi soldiers are very good choices.
-Oh. During last mission you get to control a very special soldier. His abilities are OP and have 4 turns cool down, so use them wisely.
-During last mission, whenever you kill a group of enemies and music stops, you can take a turn or two to reload ammo and help cool down your abilities.
So... pick as many abilities as you can with your main squad, level up your main squad with covert operations, give them best quality upgrades for weapons.