This is sort of a guide, but also part reminder of everything I picked up ever since I finally was able to try out all COs in depth (leveling them up past mastery). Also, only counting active stuff. Not passive stuff like Chrono Field, or Reconstruction Beam (unless it involves the mastery points, or researching this to cover 2 extra targets)
========= in general
—Get Recon Beam upgrade if your ally is going heavy on mech. Pre-game, consider putting points into that mastery
—take care of 1st wave
—take care of straggling enemies
Saves some COs a whole teleport spell
—If your ally has solid attack forces, then attach Energizers to them as a force multiplier
—Use Purifier Beam to soften up attack waves and/or fortifications
—If you have Phase Detonation researched (and you should ), it’s helpful to accompany COs who have “glass cannons”, like Raynor’s bioball, HH’s Reapers, Mengk’s Troopers, Vorazun’s units, Zagara’s units. Even stunning a portion of the enemy army will give them a leg up in battle.
You don’t even need to unload massive amounts of SoA energy into this. Just time it so that you keep maximum amount of enemies in perpetual stun (they stay stunned for 1.5s, so work out a rhythm if you want to stun multiple portions)
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========= mission specific
==== Oblivious Express
Make a few cannons or Mono + Obs to keep nuking ghosts at bay
===== Dead Of Night
If you’re going d, then focus on that. Properly taking care of d lets your ally focus on pushing. Even at night
===== Cradle Of Death
Make defense! They always pile on your main base, from all 3 entrances
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========= CO specific
===== HH
Make defenses for them. They can focus on pushing
===== Tychus
Ditto here as with above
===== Raynor
Ditto here
==== Nova
Give Chrono Boost to her production. nearly Any other Protoss CO can do this, but with Karax, even more so. Also, be sure to hit Chrono Wave often (I’m assuming your typical Nova player won’t leave their production charges maxed out)
==== Dehaka
This is assuming you have mastery in SoA start energy/max energy, take out towers early on. Early level Dehaka sucks at taking out towers. He only does 20 to 40 damage, and doesn’t really have any applicable skills vs. them. It’s like some sort of “mandatory rockslapping”. I got thanked a few times for this. The feeling’s mutual, as the quicker Dehaka can get up and running, the better it is for the both of us
bunkers = 4 shots (400hp. 550 if Amon bunkers get that fortified upgrade)… add an extra shot if you’d like to take out the lot of infantry in there (although some Dehaka players will want to Consume them)
Cannons = 3 shots 150/150
Spine Crawler = 4 shots (300hp + 2 armor)
==== Abathur
Ditto. Take out towers, try to lead units into Toxic Nests. Same principle here… the quicker his UEs get up and running, the fast it is for everyone else
===== Stukov
Snipe anti-swarmy threats (e.g. Colossi, HT, Reavers, Tanks, Banalings)
==== Artanis
Even though he has Deploy Power Field, it has a 30s cd, and can only be in one place at a time. Dropping Pylons on the field lets both of you have greater map coverage. Also give him Pylons for base use. He doesn’t need the supply. Just the power fields, so if you can coordinate that, it saves him resources and time
===== Fenix
Similar deal here. Deploy Adepts to the frontline easier. It also gives Fenix himself vision to teleport his various suits for rapid reinforcement
==== Alarak
Drop pylons for Structure Overload. He can just bring a War Prism and put it in stationary mode, but I have yet to see anybody actually do this.
==== Vorazun
If she drops a Dark Pylon in a defensive area, then make a few cannons (yours are better and have 0 bt), and Shield Batteries (they go a long way towards keeping her costly Dpylons alive).