Achilles

Question : Does Tough protect Achilles from Sabotage?

Answer : No. Unless stated otherwise, Cards in Reserve have no effect, save their Support Ability (in the colored box).

Question : If my opponent uses a Spell or ability talking about "target Character" and all my Characters are Tough, what happens?

Answer : When your opponent casts a Spell or resolve an ability that can target one of your Tough Characters, they decide if they want to pay the extra mana to make one of your Tough Character a valid target. If they choose not to (because they don't have enough mana, or because they don't want to), the effect resolves as if your Characters were untargettable.
Depending on the effect, it might mean the effect may simply do nothing, or it might mean they must target one of their own Characters.

Question : If the Tough Character of my opponent is the only possible target of my Spell of ability, do I have to pay extra mana?

Answer : No. If you choose to pay extra Mana, the Tough Character becomes a valid target. If you choose not to, and there's no valid target, the effect might do nothing. Don't forget that Characters you control may be valid targets: if you choose not to pay the extra mana (or if you can't), you may have to target them!