How do you survive the Defending the Keep mission?

Have you tried the strategy listed in the linked wiki page?

1. Pre-battle, load up on as much AOE as your profession can get.
2. Man the cannon at the top of the walkway
3. When you see a giant horde of undead coming, fire one last cannonball or grapeshot into them and then fall back. Don't wait for the NPCs to tell you to run away.
4. Do not attempt to fall back to the next canon. Move instead to the arrow launcher in the back and keep firing at the group near the NPCs.
5. When you die, don't keep rezzing repeatedly. Take one rez and AOE the biggest cluster you can find then take a break while the NPCs clear out some of the remaining undead.
6. Start rezzing and fighting again when the last NPC reaches about 10-15% health. Don't worry too much about him. He has a lot of health.
7. Once the horde has been cleared out, don't go fighting the abominations but instead start rezzing the NPCs to do that for you

http://wiki.guildwars2.com/wiki/Defending_the_Keep