Helpful Hints & Tips:
The first thing you want to do prior to entering the Mission is to make sure you have the proper skills equipped. Once the mission begins, you will not have the opportunity to change your skills until you re-enter the city of Ascalon in the Post-Searing world. Make sure you have an ample supply of healing skills for yourself and your party. The Resurrection Signet is not vital as you are Rezzed automatically in both phases of the mission, but that does take time, so you may want to have it in reserve if necessary.
As mentioned above, if you skip the PvP portion of the mission, you will have no way to go back to replay it, so it's worth the time to get in some action if you are new to PvP. At the very least, pick up some extra XP on your way to the Post-Searing part of the game, which will be a high priority, as you will encounter many higher level and tougher foes. Plus it's always fun to pound on some of the other real world players in the game as opposed to NPC's. In addition, you will be able to revist this area in Post-Searing for some more PvP action with other player characters (up to a max of level 10).
The best way to get through this mission, especially if it's your first run, is to complete the PvP phase, then log out after the intro to the second phase begins. Once you log back in, you will pick up at the intro to the second phase once again, but this time be matched with only henchman. This comes in handy after the Mission is completed, that way you are free to view the cinematic, then explore the Post-Searing world at your leisure. You will want to do this as if you remain in a party with player characters. they may want to skip the cinematic and once in Post-Searing head directly for the city, not allowing any new players to this part of the game to get the first quest from Captain Osiric (worth a nice 500 XP), and get acclimated to the new surroundings. It's unfortunate that many vets to the mission (and those who just don't care enough to take the time to enjoy every aspect of the game) can ruin the experience for others who do, and there's no way to leave the party until you reach Post-Searing Ascalon without logging out of the game.