Okay, I've got into the thankless task of trying to be a Tanky Blue Mage... Experimented with a lot of things, including melding tenacity to see if it helped with the spiky-ness.

But I realized one thing... Even if the stats/mighty guard defense/tank mimicry were tuned to make Blue Tanks more resistant, it won't do any good unless they rectify a huge issue with the role:

Mitigation spells having a cast time.

The bosses were all designed behind knowing that conventional Tank mitigation spells are insta cast and oGCD - so you can properly react to the tankbuster cast time. This is nearly impossible as a Blue Mage. Diamondback, Dragon Force and Chelonian Gate all have a nasty nearly 2s cast time, and if you're using GCDs to attack the boss, chances are the global cooldown will still be rolling when you absolutely need to press one of those buttons.

Swiftcast helps a bit, but still it won't be up for every time you need. My proposal is, just let all of those spells be instant-cast while you're under Tank mimicry! It will help the survivability a LOT.