While I'm not sure, I'm fairly certain that Obi-Wan vs. Vader is not a sure win. Vader does have a considerable advantage, but if Obi-wan uses Heal and had higher health in the beginning, he should be able to make it. What's most important is that during the fight, Vader can do nothing to anyone else, while Obi-Wan can still attack.

I'm working on a few level-specific stratagy guides, though none yet for multiplayer. Until then, you may want to check my article on Galactic Conquest on the main site: it may help you out some.

I'm curious how highly upgraded his starbases are. Anything below three should not give him much of "defensive advantage", unless you're not properly covering your Nebs with Corvettes.

The next time you attack him, send the whole fleet against whichever planet has a lower starbase, and do the following:

Send your Y and Xwings off to one side, do not move them forward yet. Move Home One, the Nebs, and the Vettes forward--keep your Corvettes literally right beside your heavier ships, not ahead, not behind, but intermixed.

Concentrate your Heavy Ship fire first on his Heavy Ships, using Ackbar's Concentrate Fire, and then retasking your corvettes from firing on the designated target, to bomber patrol. Ignore his Tie fighters.

In the midst of this attack, move your X wings and Y wings forward from the flank you left them on. Concentrate your Y-Wing fire first on his Hanger Bay--as this will deny him further reinforcements--and then on his larger ships, and finally the Shield Generator of the Space Station.

Your Xwings, meanwhile, should be tasked to keeping your Ywings safe. Attack any fighters which come near the Ywings, and sacrifice your Xwings against any Tartans which come near, until your larger ships or Vettes can end that threat.

With the space battle won, take that planet as fast as possible, so long as you have at least 1 Ywing unit still in orbit. Play a defensive battle, at first, and as you move up, save your Bombing Run until he masses forces to oppose you. If he is NOT massing anything and counterattacking, sacrifice a fast, scouting unit like the T-2b to see his base, and if it lacks a Shield Generator, aim for his heaviest factory--probably his ATST factory--to deny him reinforcements.

Without massed AT-ST's, he has absolutely nothing which can stand up to a properly micromanaged Obi-Wan. Even Vader can lose so long as you're smart with Force Protect and Force Heal. Wait to use Force Heal until Obi Wan is very damaged, and you shouldn't have a problem with Vader.

Having won those two battles, and the planet with them, you now need to keep striking hard and fast, to keep him reeling. As soon as you have recovered your Fleet losses--or even if you haven't, so long as the target is soft enough--strike at another planet from the air. If you find a soft underbelly is exposed now that you've taken his heavily defended border planet, strike hard and fast with Obi Wan and a raid, or else a full attack, to maximize the damage to his economy.

In maps like Path To Endor, the player who first knocks the other back, provided he capitalizes on the victory, should usually win. Just remember to keep knocking him hard, even if you lose ships. So long as you can defend the planets you've taken by land, it's fine if you don't have the ships to keep them all. The important thing is that he doesn't have time to force you into another Stalemate.

couldn't have said it better stormraider.

At that point in the game you cant go defensive.
You should try probing for weak points in his defences to exploit. If economically possible, Raid his planets and destroy his mining facilities, even if it means losing some units.
Get tricky with him. Attack a planet, pull a quick retreat, and attack different planet moments later. Get him to concentrate his forces somewhere and expose his defences somewhere else. Always leave him guessing where the next attack will fall, not you.