I beat the high dragon on my first visit to the Mountain, about halfway through the game. I too was an Assasin/Bard DW Rogue, with Alistair as Tank (Templar/Champion), Wynne for Healing/Support (Spirit Healer/Doesn't Really Matter) and Leliana for kicks/ranged attack (Bard/Ranger?).
The first general tip I can give you is to setup your tactics. I assume you're not putting yourself through some voluntary potion abstinence, so get your characters to drink health potions when their health is below 20%. Get Wynne to heal anyone when they get below 50%, with herself as first priority. Also make sure to keep her mana up.
Tactics alone should keep Alistair alive for a fair bit. Make sure you give him massive armor, I had Cailan's set from the RtO DLC. I considered giving him Yusaris (the +damage to Dragons blade from the Mage's Tower) but figured a shield went better. Put +damage (Frost and Lighting, not Fire) Runes on his sword, I had Maric's.
Alistair also has Rally, which is great for any of your melee attackers. But by far his best use is Shield Bash, which will free you if you've been grabbed by the dragon. Tactics may/may not work for this, so you may have to do it manually. Either way, it prevents anyone except Alistair from being picked up by the Dragon. If Alistair himself is, have Wynne heal him while he's in the air, it should keep him alive, and is generally a lot easier than reviving him if he dies. That said, if you've built him right, he should survive the chomp (albeit with very little health at the end).
Wynne should cast as many support spells as she can, and then go on healing duty. When I played, she was almost completely tactics controlled, I almost never needed to micro her. Frost Weapons, Curses on the Dragon, etc. Don't make her do damage. She will suck at it. Give her any equipment to increase her willpower and mana reserves. She's not doing any damage, so forget about that.
Leliana served basically to sing the Song of Courage. After that, she shot arrows. Give her some nice light armor, anything to improve her defense and damage (in that order, her song is far too important).
As for your main character, well...
Don't use active abilities.
Stackables don't stack on active abilities. Once you've set everything up, just go and backstab the damn thing. Each hit should see several +damage numbers.
You'll be doing more damage personally than most of your other characters combined, so you have to make it count. Wynne's support spells, Alistair's auras, Leliana's songs should give you a hefty boost.
You're an assassin, so keep Mark of Death on the Dragon at all times. Depending on your second specialization (if you have one yet), either sing a Song of Courage, or turn Dueling on.
Again, I had Duncan's Dagger, which is +10 damage against Dragons. Add some runes to my daggers, and some Crow Poison, and I'm way ahead.
You probably don't have access to the Felon's Coat yet, so Shadow of the Empire should do fine for now. It's much cheaper too.
As far as attacks go, as long as Wynne is alive and active, none of your characters should die. The only time she's personally threatened is when the Dragon lands on her. I immediately micro and tell her to move the hell away, keeping the Dragon occupied.
You should constantly focus on getting around the Dragon. It will periodically turn around and face you, and you should go around.
That should be it. As you can probably see, I played my own character a bit of a glass cannon, and got through fine, fairly quickly. A more defense oriented character may have a more leisurely - though longer - fight.
Miscellaneous tips:
If you really must, go and get that drakeskin armor. I never did, because I didn't want to go and come back to the mountain.
Position your squad before calling the dragon, it makes things start off a bit easier.
There are a couple of glitch-based tactics that you can use, but I don't think you'll want to. There's one using blizzard on the mountain, and another where you stand all the way down by the door, and take pot shots with your bow.
Stack your stackables. You'll do well over +50% damage if you do it right.
