When fixing defense for hcssf, you need to consider what skills will be used & "how" the character plays. This will dictate the scenarios you are most likely to experience, and therefore prepare for. In your case, your gear doesn't really seem to really complement the story and playstyle your skills are telling me.
You are a mercenary with a crossbow shooting gas and flash grenades with auto-stormbolts on dodge roll, and plant an oil ballista and use explosive shot to detonate your bolts and ignite the oil. You don't need to be in melee range, but that will happen just because you're mostly a "damage over time" play style with the ignites and some light burst of fire/lightning from gas explosion/stormbolts. There's a window of dodging required after attacking, and for enemies with a lot of health/resistances, you're gonna be dodge rolling a lot. This means we need to think about taking hits (mitigation), making sure we don't die to multiple hits at once (avoidance), and making sure that if we do get hit, the next hit won't kill us (recovery), and how we deal with ailments (bleed, stun, shock, ground degens etc.)
There is also the fact that it will be easier for you to get strength and dexterity than it will to get intelligence. This matters, because attributes are required for gear and gems. Their inherent bonuses aren't too important in the campaign but they're not nothing.
So lets go through it, in the context of what is available to you, a campaign merc:
Mitigation:
- Easy: Armour, resistances, sorcery ward, deflection, armour applied to elemental damage
- Awkward: ES because of attribute requirements (we are dum), damage to mana before life (low int, low mana, tough to recover mana other than flask using DoT skills)
Avoidance:
- Easy: Evasion, manually dodging, sorcery ward again - it's a bit like ES in that it fills both categories
- Awkward: Block (we hold an xbow... so this is out of the question)
Recovery:
- Easy: Regen, flask, life on kill
- Awkward: ES start of recharge and recharge rate (just hard to get for us), leech - you are using mostly grenades, so leech is kinda meh. Life on hit also has similar issue.
So with this in mind, lets consider your gear.
Your chest armour is Evasion + Energy Shield, and is very low level, and is not providing much of either. Same with your boots (AR/ES). And your gloves (Super low Evasion). Your helmet is pretty aight for the campaign. Your Amulet and Ruby ring and belt will make the cut.
So replace your chest armour, your gloves, and your boots, with all AR/EV basetype gear. Get as much AR/EV, life, and resistances as you can. Sorcery ward will also help with ele damage remember. Replace your topaz ring with a better ring for resistances and life - replace this one first to try to relieve the resistance burden on your other gear so that you can focus on AR/EV, life, on them to get sorcery ward as high as possible. It can be hard, but try.
Spec out of pitiless killer - damage over time doesn't work with culling strike and you are not hitting nearly often enough to take advantage of this type of ascendancy. It works best when your attacks are hitting very very very often, otherwise just slot the culling strike support gem on your basic bow attack to pull it out when you need it. Respec the points into sorcery ward. With all of your new AR/EV gear, this will help tremendously against elemental damage in the campaign, including the tavakai fight.
Get some regen on the tree, and if you can, get gear with regen rolls. It helps when you are not a build that leeches so you can more easily shrug off degens and survive bleed/poison. Nodes related to resisting slows and making sure you can always be mobile will help you a lot, since you use a highly mobile playstyle.
Don't worry about ES - your sorcery ward will play this roll when you spec your next ascendancy points into it, and it scales off of AR/EV.
For charms, if you end up taking the nodes that make you immune to either poison/bleed (recommend) or crystalized immunities (recommend) - fill the gaps. So stone charm likely good here to make sure you can always roll away from hits, and then either dousing/bleed/poison - whichever you need, and then for the last slot I like to use the silver charm or chaos charm.
Next is your tree:
The gist will be to spec out of a lot of your current nodes and into defensive nodes - you will likely not notice a huge difference because in the campaign, numbers on attacks rarely are large enough to begin with to make speccing into offensive nodes worth the points unless they add functionality. Things like "increased damage" will not feel that impactful to offense - but they absolutely will to defense.
You also have some overlap in functionality - your flash grenades blind, and you spend nodes for blinding on hit. Most of your skills do NOT do lightning damage, but you have lightning penetration on the tree - in these sort of instances it's usually better to put lightning pen gem straight on the skill, and use your points in a more impactful manner. Same idea for things like attack speed, or AoE - if only 1-2 of your skills will benefit, and most will not, AoE or attack speed tree nodes might not be the best choice. In your case I think it's ok.
So imo almost all of your points on the bottom right of the tree need to go. They aren't providing you as impactful benefit as other nodes might at the moment. Also the 3 you have around adrenaline rush - not worth it in campaign.
So you're gonna get like ~30 points to respec. Get the increased AR/EV nodes. Things like leather bound gauntlets, tolerant equipment - all those kind of clusters. Take defiance and impenetrable shell. Then you can go into things like the crossbow damage nodes, the grenade nodes. AoE isn't really necessary for campaign either, but it is QoL. You might be attribute strapped - that's why merc has such easy access to the attribute "star" and other clusters in that area. Use it if you have to. You can always spec out later.
I'm not sure how much mirage archer and emergency reload are doing for you. You can keep mirage archer if you switch out the chest and lose the 30 spirit you'll need for emergency reload - but I think I personally would choose wind dancer and even something like attrition. Wind dancer helps counteract the evasion penalty from sorcery ward and if you put blind and aoe, maim, whatever on it - it makes sure that when you get hit, enemies are debuffed, and less likely to hit you twice, or if they do, less likely to crit, and they are slowed.
Maybe re-examine your support gems in light of the tree adjustments. You want to make sure your elemental damage penetrates, and think about what you want each skill to do/what role it's filling, and how you - specifically you - use the skill. Specialize the supports for that role and usage. Generic supports like damage/aoe/attack speed can be fine if you're using it in a generic way, but it is worth considering all the options here. It takes some time and trial and error but it's worth it I promise - you can definitely end up in scenarios where "sheet" dps or numbers are smaller but the skill will work much better for you due to how you are using and playing with it. So do not be afraid to experiment. There are no "always correct" answers for every single player in this case, you really do need to see what you like most when you are using a variety of skills like you are.
As a final word, a better xbow will really smooth over gaps in defenses and build weaknesses lol. Whatever you choose to do, just make sure you replace pitiless killer with sorcery ward and use AR/EV gear - that will have the largest impact for you.
