A Sinister Turn

The second Prophecy Mission's achievements are much harder to get than the first one. In "A Sinister Turn" you'r normal achievement is to kill all enemy protoss which will take a lot of time and your hard achievement is to finish the level in 25 minutes which can be quite though, this pretty much means you can't do both achievements on one go.

To do the normal achievement you should just try to build as solid a force as you can possibly muster; get a few gateways and secure the robotics facility and the dark shrine early on, you can then move out and set up static defenses just outside the two enemy bases. Keep producing as many Immortals as you can in the west, if your economy can support it you should get a second robotics facility, I would also recommend having three to four gateways to produce mainly stalkers. The east side is pretty easy to take, focus down the void rays with your stalkers and it is pretty much powering through, secure the templar archives and build a nexus to increase your income.

The problem with this achievement is that the longer you take the tougher Maar will become and the protoss forces will also get stronger and more plentiful over time, but it takes time and a pretty sizable army to take down the western base so you will need to expand in the east as soon as you have taken it out. Mass stalkers and immortals and attack the west when you feel comfortable, don't use zealots here as the archons there will just destroy zealots. Try to get all your upgrades before you attack. Once you have both bases you can pretty much force your way through the final base, I used a small static defense just across the bridge from the base to fall back to when Maaw spanwed.

For the hard achievement, "Maar-ked for Death", I used two gateways to produce a few zealots and stalkers in the beginning, enough to counter Maaw the first few rounds. As soon as I could afford it I built A static defense with several (5-6) cannons just above the eastern ramp and put my zealots there. With this many cannons and a few zealots (you might have to replace the zealots once in a while) nothing will get past it so you can pretty much leave it to take care of itself (this is nice if you like me have problems with multitasking).

In the west I set up one or two cannons by the ramp and then pushed out to secure the robotics facility. A few cannons as static defense and continue producing immortals and stalkers (zealts are a waste of money here IMO). Once the force is big enough push out to attack the south protoss base; run south and hide somewhere until Maaw spawn, wait for him to pass you and then attack, try to sneak around the side so you don't have to kill the whole base and withdraw when Maaw spawns.