Any suggestions for what might make The Grey more challenging?
I do not envy your task... First, you have to find ways to outdamage dragoons at range with 2 shield healing devices. Then, you have to deal with chain lightning's stun allowing you to trivialize defenders. Then you have to worry about maelstrom turtling. Then you have to worry about reavers (since trees have low collision radius).
There's a severe difficulty spike from the too easy missions of 3-0 to 3-whatever and the excavation (dig too deep or something).
In sins of our fathers, Tempest shield heals an additional 200 shield as well as increasing current+max shield by 200, the tooltip does not mention this.
Breaking down each mission in a bit of detail (memory is fairly shaky):
Protoss holdout/micro: Losses never really mean anything, defense ends before missing units ever come into play.
Moloch infiltration: Too slow pacing. Mere stealth crawling is unentertaining and just serves to extend the game time
Boss memory dive: outside of Jepok playing in red, there isn't much you can do to lose this if you just spam your healing.
Terran jailbreak: main point of concern is that you can to the end of the nexus's exit hallway and still have to wait for enemies to spawn before leaving. I thought the point was to exit quickly, not to make sure that we get a good KDR?
The grey: healing outdoes most ranged units (melee is simple to kite), 5 second stun is incredibly powerful, all units are bio so even if you somehow get on the back foot you can just Dark Archon Maelstrom to win.
Terran infiltration: having +50 more range than marines and a slow makes post-bots trivial.