You don't need constitution at all, you simply need the right setup.
First, craft best armor (173 armor rating)
Second, get this by level 13: (build is basically done by now)
http://da-skills.net...000,00000,000,0You can get this by level 20: (7 points mostly not needed...)
http://da-skills.net...110,00111,000,0Third, get Perilous grade daggers (125+ damage)
Finally, all you need to do is follow this strategy:
When at a choke point, Flask of Fire + 4x Elemental Mines. Don't get more, you'll get a cooldown.
When Flask of Fire expires, Flask of Frost and get in the enemies' faces. Just hack at them with your daggers, use Evade when Flask of Frost expires. Flask of Frost is tremendously good at keeping you alive if you touch the enemies, use that advantage, at range you're worthless.
In between those 2 things, you just act like an Assassin. Meaning you don't attack from the front, you flank. You don't try to get the aggro, you flank.
Don't underestimate the power of Evade, not only does it get you out of bad situation but it can also be very damaging. Use it when you're touching the enemy, if you don't touch them then it's worthless.
With that setup (and a Rock Armor potion), I've tanked the Red Templar Commander while he was spinning and I killed him solo in less than 30 seconds (Perilous). Evade was doing about 2.5 to 3k every 2 seconds while I got hit for 1 damage... Of course my promotions helped, but you can get the same results as it will just take you a bit more time (1 minute for regular Perilous players?).