Dominus High Templar ,merciless...

- Try to lure it on another target with a totem. That will give you a small window to hit him.
To void his lightning stream and fist of god, leap slam is a good addition. A good move speed too.
- With 5+ att/s, you're certainly using multistrike. It's perhaps part of the problem because you are force to attack 3 times in a row, which reduces your chance to evade incoming attack.
- Your life is a bit short. 3k and more make it easier.

I'll give it a try with my lvl 71 duelist which has stats quite similar to yours :p, to check if it's a gear/level check issue or a methodology issue.

Dominus just downed at lvl 72.



Here are my stats:


As you can see, it's not high stats. First phase was easy. During second phase, I fought dominus in the lower part of the area. Using the wall and leap slam extensively to avoid the stream of lightning spirits. I died once in the middle of exploding zombies while fleeing the stream. When my flasks were almost depleted, I ran/jumped fast in the north to kill zombies to refill them. Dominus stay south so you can do that very safely.
Last phase is easy, you just have to use your seething flask carefully (better be a hallowed flask for 3 charges). Also kill whipping miscreations asap as you don't want them cast vulnerability on you. If you don't have block, kill also tentacle miscreations asap.

I didn't use any totem, but if I had once, I'd put decoy with faster casting for that specific encounter. It won't survive long but it's enough to land a few hit on Dominus.
It's also fine on cwdt for regular farming/mapping.

To avoid the stream, run/jump perpendicular to it and go hide behind the south wall.
And You may have a harder time than me to hit Dominus because Molten strike has a larger area of effect than heavy strike. I often hit him on the edge of the south wall, so it was easy for me to go under cover if the stream was launched (always used leap to flee, except when I died ^^'). Btw, multistrike proved to not be a problem, and attack speed is definitely too slow without it.

edit: of course, a topaz flask is almost mandatory to help avoid "oh sh.." situations.
leech mechanics: http://pathofexile.gamepedia.com/Leech
By level 80, you should be over 4k life, and even more if you don't have block.

150% is really low for a melee.

Here's the duelist:




Just an average/low random duelist, with some good pieces of gear and some trash ones. 204% life on tree, aiming for 250%, and maybe more from templar area if he goes far enough in leveling.