very simple, the achievement can finish in normal mode, just spam m&m and put place all around the bridge and near the missile turret.

This one is definately harder, this is what I did

1. At start, salvage that 2 bunkers at the bridge, you're going to hold your defense at the your base ramp
2. spam m&m, I went 4 barracks, 2 with reactor
3. At 11-10 minutes mark, gather all your troops push through the left exit and go all the way up straight, you'll find 2 hatchery there, kill it, then retreat your troops back to your base, I did it with about 50 m&m, but I didn't make it out alive all the way back to my base.
4. At 3 minutes mark, gather all your troops again, and push through the extact same location, they should rebuild the hatchery already, kill it, done, even if you fail the mission, you still get the achievement, you can float your CC and fly away and win like a coward.

first two mission is managable, after that well they feels unfair.