Question: how good are you at timing your Opportunities? I ask because the only thing i found lacking on Invulnerable, early on, was a bit more healing, to the point that i was comparing my Regen Sents as more invulnerable than my Invulnerable one (at the time, I could only handle one Invulnerable because, well, it just felt squishy). then i started to learn about the Sentinel mechanic, namely the Defensive Opportunity.
Breaking it down: each attack you make adds to your Opportunity bar, that slowly depletes on its own, but quickly drains if you switch targets (focus fire is key here). When the bar is Maxed a almost-glowie humm starts, and your first two attacking powers (the first two powers in the list, not the first two powers you personally picked) gain rings around them, one is Red, the other Green. No matter which of these two powers you choose to attack with, a bright ring using your color schemes will be displayed at the enemies feet, highlighting that enemy for the rest of your group. now anyone who attacks this enemy will receive a special boost: +20% Damage if an Offensive Opportunity (the red option) is taken, or 20% of damage done returned as healing if the Defensive Opportunity (the green option) is selected.
utilizing these two mechanics, you will find your damage output increased, and your survival easier. And if you group with people, your teammates will appreciate the additional damage, or the occasional health return.