Have you tried using your map and go to a wayshrine? Or push "G" to pull up the Guild Roster and right click on a members name for a small menu, then click Travel To Player.

I found mannimarco a *** too, in the end my winning tactic was to just block all the bats he sends at you, interrupt his life siphon attack and run around like a headless chicken, mostly ignored him and concentrated on dodging and killing adds, your npcs do a good job of whittling his health down, hardest part is when all npcs are holding portals back and its just you and him toe to toe, repeated former tactics with the odd poison attack thrown at him, eventually he died, but it took me at least 5 attempts, it helped that my 2nd weapon is a resto staff and I could self heal a little, but mostly that fight seems to be about survival tactics and not just straight out blast him down.

Only way to get out is to quit, leave it for a period of time to reset, then log back in , you will be outside The Harborage again.

Try doing the final Fighters Guild quest first, it will give you a nice useful reward (not saying any spoilers), then when you fight manni, don't waste stam blocking the white balls, they hardly take any dam off you anyway, kite him with a bow, then use an interrupt (like venom arrow) when he does the life steal beam of light, and bring the adds down as quickly as poss, silver shards is good for that.

You can't do him serious damage all at once, it's an endurance fight, all you can do is whittle him down, and stay alive longer than him