Actual this isn't all that hard.
Though you cant detect the targets of Phoenix fire, it does put a buff on it's targets. So change the effects of the buff so it looks like it's frost and then trigger something that:
1: Picks every unit that can be effected by the spell every .03 (other things like .1 work here, depends on how precise you want) seconds
2: Checks if they have the buff
3: Applies the speed decrease
4: Adds them to another group of units currently affected by the buff
Make another trigger that:
1: Picks every unit in the group of units affected by the buff every .03 (once again with the .1) seconds
2: Checks if they have the buff
3: If they don't it removes them from the group and returns speed to normal.
So yeah, you can use the Phoenix Fire spell. This should also be a bit easier then creating a dummy attacking unit.
If you want I can trigger this up for you