Hehehehe, it was me!
Here is the combo I used in that game.
Fireball -> Fire nova -> Rain of ice -> Frost Nova -> deep freeze.
This does serious damage.... The icy skills are put into later since I got perma forst and be able to chase down if I could not finish the guy completely. Not to mention for 20 sec after change to ice I got weapon dmg increase (Teseer mentioned above)
Reverse is not that bad as well, since fire nova is almost instant and fireball's range is huge enough to catch running demigod.
Like the original set up... Basically poster is going in order of decreasing range... fireball is a first strike as longest range... fire nova also has deceptively long range... Then extending the time the enemy dg is in range via Ice combo. Shatter will work as a finish, especially with maxed rain and freeze.
I guess personally, I would swap out frost nova, but that's a matter of personal preference perhaps. Under this setup, you actually want to go ice heavy and fire light... since fire is your opening, you don't want to scare away the enemy with heavy fire attacks and prompt a retreat before you can spring the ice trap.
Moreover, heavy ice should buy you some time in a 2v1 situation to get away (or for your back up to arrive). Just keep in mind that ice spells have a longer recast. If you don't finish off your opponent or severely weaken him, it may be time to cowboy on out of there... which is another reason to go heavy on ice in this tactic.
And again personally, i wonder if it wouldn't be prudent (especially for crowd control of 2v1, etc. situations) to drop a circle of fire after fire nova... DoT can be useful in setting up an overall trap that is difficult to escape from.
Overall nice, and props.