The most important thing is to stay outside while fighting. Bring only 1 mele with wild blow/strike/throw for braking stances and position him at the door. Position rest of characters so they can cast on Shiro/heal tank but far enough so Shiro can't reach them (he can't cross certain line). Bring Spiteful spirit, Spoil Victor, Pain inventer on different characters - Savannah heat (or other high damage ele spell) also help. Follow wiki instruction how to setup necro hero(s).
Take Leech down first (or don't go for bonus), because he hits much harder while you going for shrines. And running around shrines while Shiro follows you is fun - because he doesn't hit hard at all if you don't attack him.
I did this with H/H (with PuG we managed to kill Shiro but Leech got us while trying for bonus) and only Koss was killed once by Leech. My group was 2 necro heroes (1 ss, 1 sv), Koss with wild blow, 2 ele and 2 monk hench + me fire ele (savannah as elite) with pain inventer.
You can find more tips in
http://gw.gamewikis.org/wiki/Talk:Gate_of_Madness_%28Mission%29
and I think there is some guide for this mission or link to it also on this forum but I am not exactly sure about that.
I know I saw somewhere picture where exactly to place H/H but can't remember location - I'll try to find that link.