When my opponent plays Icathian Rain and I play Not So Fast on one of the targeted units, is the whole spell cancelled or just the damage to that one unit?

Ruling: The entire Icathian Rain spell is countered. Not So Fast counters the spell itself, not just individual instructions. If Icathian Rain targets at least one of your units, you can play Not So Fast to counter it, preventing all six instances of 2 damage regardless of what they were targeting.

Nuances:

• The condition for playing Not So Fast is that the spell must "choose a friendly unit or gear" - if even one of Icathian Rain's six damage instances targets your unit, this condition is met
• Rules about targets becoming invalid during resolution do not apply to counter effects like Not So Fast