Tank. I didn't go into a heroic at all until I had 535 defense so I was uncrittable. I think I had around 21.5k hp on my tank. Shamans basically had quest blues and titansteel guardians. So very close to what you have.

I would read the gundrak boss strats because there are gimmicks that you really need to take advantage of to make the place doable. For instance, on the snake boss, you should position your shamans on the ramp coming down from the entrance 1 level up from your tank - they wont get hit with any poison or AoE and it will make picking up the snakes with your tank easier, though I can practically kill the boss now before any snakes arrive.

Second boss (colossus) you have to use 4 healing stream totems and a nature resistance, and when you first start out it really helps a lot to drop 4x earth elementals (but only AFTER the boss transforms into an elemental).

Third boss is pretty easy no special strat other than try to interrupt his transformation with earth shock and if you can't, then make sure when he quakes you spam LHW on your tank and then once your tank is back to full THEN chain heal your shamans.

Eck (heroic only boss) is pretty easy IMO - park your shamans in a corner outside his room wedged in so Eck can't jump behind them...when he leaps on them, wait about 2 seconds, then taunt him, that way when he spits he'll be facing your tank.

Last boss is by far the toughest, I didn't kill him until my third trip in when I got some good advice - when he turns into a rhino, do not attack with your shamans at all, do nothing but heal your tank or shamans, period. Other than that, you want him to be far away from the shamans when hes in troll form, and close to them when hes in rhino form.

Of course all this only works if your tank is uncrittable, because unfortunately some of these bosses will 1 shot your tank if he gets crit (like the last boss, the third boss, etc).

Hopefully that advice helps, once you learn the tricks I still think gundrak is the easiest instance even moreso than UK.