It really depends on your build as to which devotions are the strongest defensively.
I’ve created several 2H Melee tanks that can’t be killed by putting 20 points in blue and 10 points in yellow.
For 45 devotion points you can take:
lizard
sailor’s guide
eel
lion
dryad
tortoise
solemn watcher
tree of life
ishtak, the spring maiden
light of empyrion
And you will still have 10 points left over to spend wherever. 8 points if you max out tree of life.
You say it as if i’m sacrificing a huge portion of damage. In fact, i give up no more than 20% total damage for that. It isnt THAT critical.
Sure. Okay.
But at the end of the day, the empirical data is there
Time dilation imo. Double mirror + Mirror Mark cycles > every single devotion proc in the game 
One on one, yes. But not when fighting multiple bosses in SR
Why wouldn’t mirror/MoT combo be effective in mobs?
What’s mirror? Arcanist ability (can you tell I don’t play that mastery)?
Yes, Arcanist ability. Time Dilation can reset your abilities. Mirror makes you practically invulnerable for short period. With Time Dilation you can use two straight mirrors. Spellbinders have also Mark of Torment. So it’s a defensive classic in GD- Mark, Mirror and Time dilation from Aeon as reset.
Also this devotion works well with Oathkeeper builds, since it resets your Ascension.
It also resets skills like overguard.
But be careful of arcane mobs, because they will dispel these skills
Mirror is fine, but mark is single target AFAIK, so if you are attacked by Kymon and Reaper, choose your fate. I’d rather invest in overall toughness with good armor, resists, regen (or reliable ADCTH). But that’s me, and I’m a chicken 
What’s wrong with it being a single target skill? You cast it on kymon and be like MC hammer for 5-6s because they #canttouchthis
This is probably one of the most common mechanics glass cannons use to blow up priority targets.
I.e. You want to kill something dangerous in those 5-6 seconds.
And that duration goes up to 8 - 9s with mirror, and almost doubles with aeon’s.
Which gives you plenty of time to facetank a nem horde.
What about Reaper? Or any SR situation when you get attacked by multiple hard hitting mobs at the same time?
In SC I’d rely on repositioning and this combo would be perfect. And if you misapplied your MoT once in a while, who cares, right?
If you click the link, you’ll see a gif where I facetank kymon’s chestplosion, kuba, and ekket’zul in the green spawn zone in naked crucible.
https://www.grimtools.com/calc/lNk68dRZ
That’s the GT - it’s a low armor, low phys res, lowish DA build which relies on massive heals/adcth to tank.
Basically, the build involves cycling through MoT, haunted steel, and ghoul to stay alive.
Can you put money on this that you would run 10/10 of these and never die?
To make the explanation easier, let’s call this playstyle “active defence.”
Whether you live or not depends on many factors:
how low are you when you MoT? Obviously if you do it with a silver of health you’re gonna get killed
Is this SR where there are dispels?
Did you MoT the right target? Because the worst thing is casting it on some trash monster, and then killing it the very next second.
Do you have an exit strategy? e.g. if you cast MoT in a middle of a disrupt pool, you’re basically shooting yourself in the foot because you literally can’t dash/TP out of the mob which is surrounding you. You can also get killed during the casting animation of MoT/mirror itself.
So I find the best way to use active defences is to use them pre-emptively, and aggressively. That way I’m in control because I’m not waiting for an “OH SHIT” moment to use it which will most likely get me killed.
Say I’ve reacted too late to aleksander’s meteor. MoT will let you shrug it off.
Say you want to blow up MQ when she has her red aura on because enemy healers are coming. MoT/mirror will let you do just that.
Another thing you need to do to pull it off consistently, is to back tf up when you’re down to 0.5s of invulnerability.
That way you give yourself some breathing room.
It’s probably also worth mentioning that it’ll probably lose its effectiveness past SR 80 where everything is just RNG.
It’s also nice to have a safety net like ghoul or serenity just in case of piloting error.
@Contragor: Just occurred to me that I forgot to mention one other thing.
Let’s say this " [] " is my spec, and " X " this is the priority target I’m attacking. Let “+” be some trash hero monster.
X <---- [] ++++
The arrow indicates my line of fire.
Sometimes, what if you’re attacking a nem which spawns a tonne of adds like reaper or zantarin, it’s quite difficult to MoT the nem instead of the spawns.
So what I do is I MoT a tankier target behind me, who is outside my line of fire. That way I don’t need to worry about killing it, and I don’t need to waste precious seconds trying to MoT the nem.
It really doesnt matter, absorb happens regardless how many mobs are around you. 80% reduction is still 80% reduction, 100% is still 100%.
The only counter is Gravathul and thats about it.
Also with time dilation you can selectively mirror just for meteor/RR/mechanics from boss and ignore them completely, this beats every other defense in the game.
Arcanist with mirror survives aleks meteor + his RR + Anasteria debuff + her meteor + Theodin multi meteor. A full defensive spec retaliation warlord gets obliterated immediately.
Your build becomes more skill intensive with timings and CD management but i never minded it.
All true. The caveat is that MoT is a single target spell. And while I fully agree that with good skills, awareness, timing, you can be nearly immune to oneshots, this approach only works in softcore. I have a couple HC casters like that, but they are used for dancing and bragging not farming. And I can easily rebuild them when they RIP.
What’s so caveaty about that?
Full circle. Two or more fat targets, or one fat target surrounded by lots of trash mobs. You never misapply the spell, right? I must be very clumsy, because I do. Rarely but I do. And then I sometimes RIP.
Yeah. I never ever use MoT or try to face tank if there’s a silver of doubt that I can’t MoT the right target.
Sometimes I’ll even MoT from a distance before gap closing just to make sure I got the right target