OK so there's a lot going on here. I'll try to keep it simple cause it confuses even me at times.
Mycosynth Lattice makes all cards artifact... Even the ones that aren't played yet. So you can now cast a card that would normally be, say, 2 red and 2 colorless for 4 colorless mana or 4 mana of any color. Fist of sun's makes all cards cast 1 of each color... Which is a total cmc of 5. When combined with mycosynth it now makes a say, 9 or 10 or 12 or whatever cmc card only 5 mana of any color (Artifacts only, I believe) this effectively lowers the casting cost of other Artifacts like,
darksteel forge. Dark steel forge makes all your Artifacts indestructable.
Creeping corrosion destroys all artifacts... Which by now, all cards belonging to all players are Artifacts... But yours have indestructable so when you cast
creeping corrosion, everything you don't control, including lands, gets destroyed and everything you do control stays right where it is.