Heh heh. It is rather easy, but some missions are harder than others. It is meant to be doable on hard, and to an RTS vet fresh out of warcraft 3's insane custom maps, it is easy. However bear in mind that it will be 'hard' to a newb.
If you are looking for a REAL challenge, go online or play on insane
Bioball thats all u need, no goliaths, medic marine and marauders, just upgrade them and you'll steam roll them. Once i started doing that it was easy. Plus stim helps and medic upgrades in the armory.
yeah that one took me a long time to get past, i floated everything to the bottom right corner, had 6 vikings (mercs) and a bunch of banshee, waited till the fire purged almost everything, then swooped in at the last moment, cause the fire stoppes in front of the temple for a while, and gives u time to blow it up.