Can I copy a creature spell?

Yes. You can copy them. In addition, since you are copying a spell on the stack, you can copy things that a creature-copy normally can't. Like the value chosen for X (so your Lightning Serpent token gets counters) and kicker (so your Desolation Giant token won't have to destroy itself).

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Can clever impersonator copy an enchantment?

Yes. See the rulings quoted above. and finally, what about if I copy an enchantment creature that has been bestowed onto another creature? It doesn't copy the effect that makes it an Aura, so it enters in the regular Creature Enchantment form, not bestowed. Is clever impersonator a creature? Clever Impersonator is not a token. In that case, Clever Impersonator enters the battlefield as a 0/0 creature, and it's put into the graveyard immediately (unless something is raising its toughness above 0).

Do auras leave the battlefield?

From the basic rulebook: Some enchantments are Auras. An Aura enters the battlefield attached to a permanent and affects that permanent while it's on the battlefield. If the enchanted permanent leaves the battlefield, the Aura is put into its owner's graveyard. Consequently, do 1 1 counters stay on exiled creatures? counters are not "removed"; they simply cease to exist.

What happens to counters when a creature is exiled?

I don't remember where the actual rule is, but I know that counters DO get removed whenever the card changes zones. Also, a token that becomes exiled cannot return to the game at a later point. It no longer exists IN OR OUT of the game. Is equip instant speed? The Equip ability is done at sorcery speed. If an equipment has anither ability that lets it be attached at instant speed then yes. Equip [cost] ([Cost]: Attach this permanent to target creature you control. Activate this ability only any time you could cast a sorcery.)

And another question, can you equip lightning greaves in response?

You can just move the Greaves around in response to any targeting that happens, and the next time you receive priority (after the spell is countered by game rules) move em around again, if you need to.