The extra HP might help a sage survive mass pk as you don't have bell+magic marrow combo to prevent yourself from losing Pdef with marrow. This makes you a pretty easy target for aps archers or even barbs sins and other bms. If you are getting AA'd and it's all magic damage, sure - you would tank it better than a demon in the same gear. But sage takes a big hit to pdef to boost their mdef so taking both types of damage at the same time would be nasty.
Learn to double leap out of range of attacks, and back in with a nice stun. Leaping has saved me from zerks and SoG and other potent CC just cause it's lucky like that. But the main reason double leap is good is it can interupt skills being cast on you by a whole bunch of AAing casters/archers, or cause people to chase after you. Leap will be back well before they get close, and hopefully roar as well. Just be careful not to leap too far out of combat/away from your EP/into enemy lines - you need to stay in the fight as much as possible, but break range when necessary so you arent a sitting duck
I don't really play sage but I'm curious what they could be useful for. I think they might be fun in 1v1's except against archers. That seems to be their achilles heel is dealing with magic and physical damage at the same time. In Mass PK, I expect to get hit by both.
Make sure you get a dragon cape and vit tome if you are vit, btw.
Also, in rank 8 with the base 15 percent magic damage reduced, you can get a higher 'reduced magical damage' mod than phys, not to say you can't get a decent base amount with dragon cape etc.
Honestly on a sage BM i would assume that "reduced physical damage" and "physical defense +10%" might be even more valuable then "reduced magical damage" and "elemental resistance +10%". Not to say those aren't still great stats for tanky r8, but on my Demon BM i am lacking in reduced physical mods, and my defense with marrow/bell is pretty flat. So even a demon BM can EASILY take more damage from physical damage than from magic. I have 37%ish magic damage reduced from my r8 gear and weapon but I can only go up to about 18-20% reduced physical damage. My defense/resistances are pretty damn similar, so even in heavy armor once i use bell/marrow i tank magic damage better than i tank phys. (Which honestly works pretty well, ie i can stun in melee but those archers standing far away are only hitting me for 3k crits).
Make sure you get a dex genie, its arguably better suited for tankiness than str. Fortify is nice if you want to abuse Iron guard then use bodhi before it wears off, pretty tanky even if you're naked with a fortified IG
. Wind shield will actually save your life too. Big fan of skills that don't cost alot of energy that are defensive. I rarely use windshield for the atk speed and more for the reduced damage, but on sage i think it's better than cyclone heel if you do ever use fists, which you should so you can attack faster with pole, which purges.
Before I go off on another tangent my last reccomendation for trying vit/sage bm (even though ive never tried it). Keep the stats for all weapon types. Or at the very least get sword and pole stats, I think they require over 102 dex but I'm not sure.