This generally depends on what you want your Froslass to do. Here are a few:
Froslass@Focus Sash
Timid nature (+Spd -Atk)
EVs 252 Speed 252 Sp.Attack 4 HP
Ice Beam
Thunderbolt
Shadow ball
Destiny Bond
The idea is to attack your foes with the attack moves and then when you cling on to life with focus sash, use destiny bond to make the foe faint as well.
Froslass@Leftovers
Adamant nature (+Atk -Sp.Atk)
252 Attack 252 HP 6 Speed
Spikes
Crunch
Ice fang
Thunder Wave/Taunt/Block/Protect
Slightly more defensive physical Froslass. Speed EVs aren't really needed due to Thunder Wave, so you could replace those with Def or Sp.Def.
Froslass@Liechi Berry
Jolly nature (+Spd -Sp.Atk)
252 Speed 252 Attack 4 HP
Substitute
Ice fang
Crunch
Destiny Bond
With this one, you sub until you have 1 HP left and then get an attack boost. Either attack if you think you will OHKO the foe, or Destiny Bond if you think the foe could hang on.
Froslass@Petaya Berry
Timid nature (+Spd -Atk)
252 Speed 252 Sp.Attack 4 HP
Substitute
Ice beam
Thunderbolt/Psychic/Shadow Ball
Destiny Bond
Same as the physical Froslass with Sub, but special. Probably best to go with this one when using this method.