I’ve watched old guides of the mission and there have clearly been unstated changes to it on Hard. The enemy waves have changed to the point that it is no longer possible to acquire the “Purity of Purpose” mastery achievement (there are no videos on youtube uploaded post update 5.0.0 that show it is possible to get it). Some change that was made to the map on 7/24/2020 has made the achievement impossible. Please fix it.
Have you tried the Spectre wall strategy? Enemy has no detectors on that mission so you can make a wall of Spectres to keep most ground units out. They must be set on hold position and hold fire mode.
Also saw a comment recommending mass Vikings and Banshees. Either way this is an extremely difficult achievement so be prepared to reload saves… a lot.
Spectres DO seem to help, but the problem is I literally can’t make enough of them and then battlecruisers/carriers/brood lords/mutas/banshees wreck me regardless. Plus the splash damage from their siege tanks/colossi still kills my spectres. The problem is that there was an unknown update to the map on 7/24/2020 that changed something that makes the attacks faster/bigger.
I did it, but the achievement didn’t pop up? I checked my profile and I DO have it, but the achievement animation didn’t play on the stats screen. That seems like a bug too.