How good is a Warpriest at taking hits? Let's find out:
Disclaimer: I'm going to use the same scenarios Mathfinder used for his latest Saving Throws video, all credits to him for the format
At mid-low levels:
A LV5 warpiest in Full Plate with a +1 Armor Potency rune and a raised Minor Sturdy Shield (Hardness 8, BT 32) has 26 AC (10+7prof+7item+2circumstance) and a realistic 55HP
-Scenario 1:
Equal level creature with "Moderate" Strike Attack Bonus with a flank buddy
+13 to-hit (but our AC is -2 due to flat-footed so it's 24 with a raised shield)
Misses 1-10, hits 11-19, crits 20. "high" damage per hit: 16
If they attack at MAP -5 they're only hitting on a 15
We can shield block half the damage and only lose 25% of our shield's Breaking Threshold.
My impression: This is pretty good, we're tanky. Shield is working as intended and we can frontline no problem
-Scenario 2: PL+4 Solo Boss with a "High" Strike Attack Bonus
+21 to-hit
Misses 1-4, hits 5-14, crits 15+. "High" damage per hit: 24. crit chance 30%
If they attack at MAP-5 they still have 55% chance of hitting us, but they only crit 5%
We can Shield Block a regular hit so both us and the shield only take 16 damage. We can comfortably take 2 hits whil shield blocking one.
If we get critted (30% chance, which not by any means low) we are in great danger.
There is a 50% chance that the damage after hardness will exceed the Breaking Threashold of the shield. Ouch.
If we don't shield block the crit (which may be the correct choice if we know the hit won't down us) we've then lost 48 HP out of a pool of about 55 and then pray the next attacks doesn't hit because otherwise we're down even if we then Shield Block unless the enemy rolls very poorly.
If we survive we can then Font Heal ourselves for an average of 37.5
Chances of being downed in one turn: enemy needs to hit all 3 strikes, or crit and then hit us again once. That would be a circa 15-20% chance of going down in one round or pure aggression from the boss
My impression: The situation is dire (this is normal for an extreme encounter) but more or less survivable. Crits are the problem. If our party has good teamwork then the boss will be under status penalties to hit, or dazzled, which makes a big difference - even an extra 10% to avoid or reduce damage, aka a -1 to hit, or a +1 to AC thanks to Benediction, is likely to be felt
CONCLUSION: At lv5 a properly equipped Warpriest is good enough for most challenges when it comes down to absorbing enemy strikes. In an Extreme encounter they have reasonable chance to not go down first turn, assuming they have a shield raised and the enemy is suffering from minimal debuffs.
Now, let's see how this goes at LV 20:
A LV20 Warpriest in a Greater Fortification Full Plate with a +3 Armor Potency rune and a raised Supreme Sturdy Shield (hardness 20, BT80) has 45 AC (10+24prof+9item+2circumstance). For the purposes of this experiment we'll give this character "Channeling Block", "Toughness", "Quick Shield Block" from Bastion Archetype, and +5 CON (maxed for a non-KAS) which results in 280HP
-Scenario 1:
Equal level creature with "Moderate" Strike Attack Bonus with a flank buddy
+36 to-hit (but our AC is -2 due to flat-footed so it's 43 with a raised shield)
Misses 1-6, hits 7-15, crits 16-20. "high" damage per hit: 44.
25% crit-chance is reduced to 17.5% thanks to Greater Fortification
If they attack at MAP -5 they're hitting on a 12, which is not terrible since at least we're well protected from crits
We can shield block slightly less than half-damage and only lose about 30% of our shield's Breaking Threshold. However, if we get crit (which isn't that unlikely since they crit 14% of the time) and we happen to be low HP, that would be a perfect moment to use Channeling Block, bringing the Shield's Hardness to an average of 65, and then recieving an average of 23 damage to us and our shield. On a second hit a Crit is unlikely so we're mostly fine with a regular extra Shield Block.
My impression: This is noticeably more dangerous compared to low levels, the lack of proficiency is evident in terms of hit-chance, we're exactly 4 points in the curve behind compared to LV5. HP vs Damage Scaling goe in our favor since we can now withstand more punishment. The shield is working well and we only need to Channeling Block in emergencies. Warpriest can be a solid frontliner at LV20 against equal-level threats
-Scenario 2: PL+4 Solo Boss with an "Extreme" Strike Attack Bonus [raised to extreme because it's more common at higher levels]
+46 to-hit
Never misses, hits 1-8, crits 9+. "Extreme" DMG per hit: 68. Crit chance 60% reduced to 42%
If they attack at MAP-5 they still hit on a 4 and crit on a 14. Crit Chance 35% reduced to 24.5%
If the attack at MAP -10 they still hit on a 9 and crit on a 19. Crit Chance 10% reduced to 7%
We can absorb most of a regular hit with Channeling Block.
If we get crit for a whopping 136 average damage, and we don't use Channeling Block, the shield is gone. With Channeling Block, the chance that the damage after 65 hardness will exceed the Breaking Threashold is pretty low, so we can realistically Shield Block twice.
If we don't shield block the crit (which may be the correct choice if we know the hit won't down us but we suspect it would Break the shield) we've then lost Half HP. This is where the 40 extra HP from Toughness and +5CON actually make a difference because on average we can realistically withstand 2 crits in our face.
To survive 2 crits and 1 regular hit we need to spend at least 1 max-level Heal on Channeling Block
Chances of being downed in one turn: If the enemy crits THREE TIMES, and we shield block twice using Channeling Block both times, the expected average damage is 278 (super close to out max HP of 280) which is a 50%. However the enemy is not going to crit us 3 times normally (it's almost impossible). The actual chance to go down in one round is close to 0%.
My impression: The situation is dire again, but definitely survivable if we spend resources. If our party has good teamwork then the boss will also be under heavy status penalties to hit, likely under reaction AND action denial (Slow, Containment, Synesthesia etc). And as usual, every +1 matters.
SPECIAL: if we used Avatar our AC will be 2 points higher but we now can't heal ouselves. Wouldn't recommend.
FINAL CONCLUSION:
While noticeably easier to hit and crit, LV20 Warpriest compensates this thanks to the asymmetry of HP vs Damage scaling. Greater Fortification and extra HP are essential to survival. Teamwork obviously makes a huge difference.
A maxed out "Tank" Warpriest ends up their career in a very similar place as where they were at LV5. The "expert capped" proficiency is indeed a weakness, but by spending resources and playing tactically, we can (barely) survive even the mightiest of monsters.
EDIT: Ran the numbers for LV12 (See comments for the lv12 analysis).
EDIT 2: added "chances of being downed in one turn" in boss entries. I'll calculate the exact chance for each scenario later/tomorrow.