Do them on brutal instead of normal. Some of them are extremely hard IMO. I have only done the Zerg ones, but even as a master Zerg player they were hard.
Also, don’t use the master archives. Play through again so you can’t make broodlords when you aren’t supposed to be able to. At least, I’m pretty sure I wasn’t supposed to have Kerrigan leveled to the level she was at on some missions, so I restarted the campaign.
Just started on Protoss and the first mission where you have to kill the Zerg base is insanely hard IMO on brutal. Some are easy, but some are downright hard.