I don't think Gandalf has much chance of losing many battles. Being an Ainur and all. But his only downfall will be his pacifist ness. But all the same...
Gandalf the grey Vs The entire cast of Harry Potter
The out come?
Gandalf wins in 3.5 seconds.
OR
Sauron Vs That bad guy from Harry Potter
The out come?
Sauron did not even realise there was a battle going on and
'that bad guy from Harry potter' bursts into flames at the mere presence of Sauron's evil.
No offence to any Harry Potter fans.
Now, whom could Melkor fight?