You could look at
IDF.
Combined with Defiance and a high Con it gives you super-invul and you can also buff your team with it.
IDF will help you (and your team) shrug off waves of small attacks which virtually always precede the fights where you really need your Defiance stacks - allowing you to quickly build Defiance stacks to take you from ~ 15% damage resist to ~ 90%. You want to have probably at least 250 Con. 400ish wouldn't hurt either.
If you take IDF you can't have a damage buff like Enrage or Form of the Tempest up at the same time. Everyone wants to do more damage, and damage is agro, which is always one of my main considerations. Having said that.. I think IDF may generate agro too due to you are usually buffing at least someone from the team with it but I don't know. In my experience if someone can steal my Defiance/IDF tank agro, they can deal with it.
If you take Invul there is no point in taking IDF.
If you want to stick with Defiance but don't want to take IDF...
Block. When you first rush into an Alert, room, whatever situation that might get you faceplanted before you get your Defiance stacks up, run in and grab as much agro as you can and then .. Block until your Defiance stacks quickly build to max.
If you can click a device or two before rushing a room or boss to help buy you time to build your stacks this can help too.
You could fire off a HoT and an Active Defense like Unbreakable and Masterful Dodge before rushing a room... My tank has Defiance + IDF and I still do this all the time.
Or any combination of all of the above. Whatever buys you the seconds it takes to build those stacks.
Defiance alone with max stacks and a high con will still give higher overall damage resistance than Invul, but, Invul's damage absorption can shrug off the smaller guys easier unless you take IDF due to you take little or no damage at all from the smaller attacks with IDF's damage absorbtion. Plus, IDF generally makes it pretty simple to get your Defiance stacks maxed. IDF must be a great buff to share with squishies in your team in some situations too but I never heard anyone comment.