Discord is a spirit, or in other words a god, so he nearly can't be defeated, exempt by the Elements of Harmony, which focus a force within the Mane 6. Even when he is using tricks all the time he has got very powerful Abilities. For example in S2E2 he raises and sets sun and moon at an immense speed. I doubt that Celestia and Luna together could perform this and don't seem to be tired in any way after it. Discord never gets tired, so his powers are so huge that he can do about everything. He can probably bend time either, even when we haven't seen this yet.
Celestia's and Luna's powers don't come even close to Discord's. Nightmare Moon is probably stronger than Celestia (S4E1/2), but we don't know about Luna, because we haven't seen a lot from Celestia. Celestia just sends the Mane 6 if there is any kind of trouble and doesn't sorts it out by herself. Either she is far too strong to bother (but she is still a lot weaker than Discord), or she hasn't got that much power (either she never had, or she has lost a great part of the ones she had earlier). So Luna could be a lot stronger than her.
If we go into physics now, we see that the sun is a lot heavier than the moon, but the sun is one star, luna raises lots of stars that may be a lot larger than the sun, so she would have to use much more power. Well, Celestia rose the moon and the stars while Luna was banished, but this could be the reason why she hasn't got much power anymore. She could have used too much power during this thousand years, so she is a lot weaker now.