[0.3.1] Mortar Cannon Tactician - how to improve survivability

Hi there.
So I'm playing with Mortar Cannon Tactician, farming T15/T16 maps. I feel like I'm dealing enough damage. Therefore, I'd think my character should be fairly tanky, as many nodes in my skill tree are focused on defense. I have around 2500 HP, capped resistances, and 27k armor (which gives me 89% physical damage reduction and 85% elemental damage reduction). I also have a 42% deflection chance with a 40% damage reduction. My Mirrage Archer stuns, pins, and blinds. Wind Dancer does the same (and also knockbacks and maims). Cluster Grenade is also configured for utility. I have a lot of pins. I also have Life Leech on my ring and crossbow. Despite this, I die quite often. Sometimes I can understand this, because the explosions all over the screen completely block my view, and I simply collapse because I'm stuck in a group of mobs, further supported by strong elite attacks and DOTs on the ground. Sometimes, however, more frustratingly, I get one-shot by regular bosses at the end of the map. These are both physical attacks (a blow with a stone column), as well as a blast of fire or a single projectile.
So, is there any way to significantly increase survivability? I've seen builds with Blasphemy or Companion. Although I don't have enough Spirit for that, and I'd have to give up some defensive layer.
Link to my build: https://poe.ninja/poe2/profile/Jack0o-5773/character/JackNotBlack
I don't know why it's showing low resistances - I have capped resistances in the game.