pigletto: It depends on the item. First of all, the enemies have to be killed, nothing else. So

(and others) won't help you.
And the second problem is that they have to be killed by the same source, for example one bomb, or one wind ball or one fireball, ... So if you get

or

and it hits one enemy per shot, you won't get more than one letter. If one shot hits two enemies, you get two letters. Usually, killing many enemies together in attempt to get more letters is easier with bombs than with items (there are, of course, exceptions

)
Oh, and afaik

doesn't give more letters either.