How do you increase Agony Resistance in Fractal of the Mist at 40+ difficulty?

At this point in time you can only get a max of 30 Agony Resistance (AR) which is insufficient for surviving the Agony that hits you when fighting the Jade Maw boss. I can only imagine this getting worse as you approach Fractal of the Mists levels 50-60 where AR won't help at all.

What strategies and tactics are there for getting past level 40 difficulty?

It is very hard, but apparanetly it is possible, as you can check in the following screenshot (link here):

According to the picture owner, the maximum level is 80, you can get pass 80, but if you leave it, it will reset to 80.

At level 40, Agony does 72% HP/Sec with no resistance. At level 50, 60, 70, and 80, agony continues to deal 72% HP/Sec with no resistance. The exception to this is Jade Maw. At level 50 and above, Jade Maw's agony deals 1172% HP/sec (source). So the only boss that seems to be really hard in 40+ levels is Jade Maw, as the others you can find some ways to avoid Agony.

According to this guide, there are a few tricks you can use to survive.

• Revive orb – 250 gems – everyone dies and get ported to checkpoint. When running back, have one person go first to the Jade Maw area and die. This person then use the revive orb to get back to life. Everyone else in the party can now come to the Jade Maw area and die next to the revive orb person to get rezzed.
• Ranger pets – everyone die to agony and run back. Ranger stay back from the Jade Maw area and use Search and Rescue from a safe distance to rez people as they die again to agony while re-entering Jade Maw’ area.
• Have one person die on purpose near the left corner of the Jade Maw area (safe from all the tentacles) in P1. Rez this person to 95% HP and leave them there. Have a one or two people kill the last tentacle in P1 to trigger the agony while rest of the group stand on top of the dead group member and rez them when the agony hits. This newly rezzed person won’t get hit by agony and can rez rest of the group. Do the same for P2->P3 agony.

In the other bosses case, I would recommend the reading of this guide and its useful comments