Well Here is some info that might help.
Marth: His Up B could be used as a offensive move when your opponents have high damage. Try using his Meteor Smash Often....Just don't suicide. Another tip is that if someone sees you countering a lot, people might think your spamming.:eek:
Kirby: His Forward B (hammer) is useful. Try to stay in the air when you opponents have low damage this is also similar to Jigglypuff. Using Regular B could be helpful if the opponent(s) have some useful move.
Donkey Kong: Well his Forward B + B is a STAB when the opponent(s) has high damage.
When using a Meteor Smash try to stick with Jump Forward A ( personally i think Jump Forward A is harder) There is really not much else to say since I can't play Donkey Kong:D
I play Marth best so i should be able to help you with him, but his Meteor Smash is harder to do in Brawl. Kirby can be a ground attacker or an ariel attacker I just like using his ariel moves.
Well, Hope I helped!;)