Even if the damage dealers are focusing on a single target, your healer will get aggro on everything whenever he heals you, so you need to either Flash or target cycle often enough to keep the attention of all the other mobs. If it really comes down to it, it's better to lose that targeted mob to the over-geared ninja than to lose the others to your healer. The DoW damage dealers are sturdy enough to take a hit or two from any except a few of the final boss mobs. (This from the point of view of someone who hasn't tanked much, only to level 26, but has played Ninja to 50 and has both of the original healing jobs nearing that. I know which of those I'd rather be on for dealing with unwanted aggro.)

The level sync in this game does seem somewhat over-generous in the stats level-synced players and gear retain, so this is a somewhat common problem with on-level tanks. It's hard to compete for aggro with people who significantly out-gear you. To have any chance of keeping up, you'll need to make sure you have the best gear you can get for your level, particularly when it comes to your weapon. (The strength buffs on everything else help as well, but your weapon's damage stat is the biggest factor.)