Countering a spell means putting the spell card into its owner’s graveyard, unless an effect specifically puts it somewhere else (like Remand). 112.3. A creature spell is an actual creature with power/toughness and it has the "creature" super type right on the card. The target is the thing or creature as a whole.
This goes with the simplification of the rules for combat, we don't have things like facing and the targeting and damaging of specific body parts. But, a spell that says it affects a creature "if it is black" can be played even if there is no black creature (that part of the effect would simply do nothing).
In that case, the owner of the spell is the player under whose control it was put on the stack. If you target a creature, it's the whole creature and that would include what he's wearing. The fact that a card specifies “target creature spell” just means you can’t use it on other kinds of spells that aren’t creatures. For instance, a spell that says "target black creature" can only be played if there is a black creature to play it on. A spell’s controller is, by default, the player who put it on the stack.
See also rule 115.4. Every spell has a controller. Your spell that buffed your creatures was not a legal target for his spell that can only target creature spells while they're on the stack. As long as it is a non-Demon, it is fine. Only permanents are legal targets for spells and abilities, unless a spell or ability (a) specifies that it can target an object in another zone or a player, or (b) targets an object that can’t exist on the battlefield, such as a spell or ability. It doesn't matter if you can't actually destroy the creature. No. Just as a note, the only targeting restrictions a spell or ability has is what is written after the word "target". However unless otherwise stated, creatures take the damage not the items or clothing.. A spell’s owner is the same as the owner of the card that represents it, unless it’s a copy. In Reaper from the Abyss's case, you must target a "non-Demon creature". Your friend tried to counter what was either an instant or a sorcery, the "creatures get x until end of turn" whatever spell that you cast.