It seems that Celestial Alignment will effectively work like Naga Sea Witch does with Giants, reducing the cost to 1 in-hand, which then gets set to 0 by the Giant's effect. It also has the upside of making Ultimate Infestation and Overflow cost 1 and locks the opponent out of two-card responses (Wild Pyromancer/Equality, for example), or out of any real response with Loatheb. Subsequently, here's a decklist to try to abuse this! I added in Oaken Summons with Injured Marauder, as that thing stops aggro cold. I think Frost Giant is actually better than Molten Giant, as you might not have taken any damage by the time you play Celestial Alignment if you're against a slower deck or got a bunch of early, good ramp.

I certainly know what I'll be playing Day 1, that's for sure!