lol, you don't frost the centre of a tower farm! You auto attack the closest tower, then frost, then rain of ice the centre of the farm (hopefully catching rook for 800damage too, and MOST DEFINITELY WAIT for a minion wave to be moving through his farm too for the xp).
If you jump into the centre of a farm you're asking for a UB-TB-Rook sandwich.
Then fire mode, give Rook a tasty fireball, drop back to frost. Autto attack the first tower down, then step forward and nova 2nd row....
The idea is to push rook and his farm slowly in the other direction, much the same way as he builds it in your direction.
You just gotta watch for his teammates tele to his towers, and as you knock them down of course the rook will raise more, but the idea is to push his frontline back, whilst still gaining creep exp and grinding rook's mana and hp down... It's a slower process than just WHAM BAM, unless you're dealing with noobs then goodie goodie!