There is nothing you can do. It is simple. Eevee can't evolve into Glaceon at level 100. It needs to be level 99 and below in order to do this.
Breed a new one using a Ditto which you can find in Giant Chasm. Since he cantt evolve at level 100. After that you follow this video:
It is simple to get a Glaceon with a eevee.
Capture an eevee in the wild. (Can be found in Castelia city)
If eevee is a Male, then breed it with a ditto or you wont get a new eevee
If eevee is a Female, Then breed it with A male Pokemon in the same egg group that eevee is in, Field, or a ditto
if eevee is Lv. 1-99, you can evolve it.
Once you have followed these steps you go to either near icy rock in twist mountain. Then have a Pokemon battle with the eevee. Or if you have a different game from Pokemon Black/White 1/2 then go to route 217 and go to icy rock. In the end you have a Pokemon battle next to icy rock once you have found it.