I’d put Hijikata in tier 3 personally, but I am of the opinion that his gimmick is nothing more than an bonus, not something you try to actively use.
Consider the opportunity cost to make it work: on top of what all Buster crit Zerkers want (stars, attack and Buster buffs, crit buffs, and a way to keep them alive until the enemies are all dead), he specifically wants to get to low HP and then stay there without dieing. This is near impossible without targetable guts, so you’re forced to run Quetz, Paracelsus or whatever. Now you’re actually running Hiji, a regular support that provides the usual crit zerker dish (Merlin for example) and a second servant that does nothing but Guts for Hiji (Quetz at least has a Buster buff too, but she also steals some his stars because his star gather skill is slightly worse than Eternal Arms Mastery). And just having Guts means Hiji will die the next time he’s hit; a full HP Hiji can at least take a hit or two.
Compare this to simply running two proper supports that provide Hiji with damage buffs, stars and survivability, similar to what you would do for Lancelot and Raikou (outside of single-use NP nuking). To make use of Hiji’s gimmick, you give up a chunk of damage and survivability a proper second support would give you, and in exchange you get bonus damage for him and rely on playing around an extremely risky and inconsistent gimmick.
I’m convinced you get way better mileage out of him by simply treating him the same as you would any other critzerker and think of his low HP gimmick as a free bonus if he does get hit. As a buster-focused 5* Berserker with in-kit Buster and crit buffs as well as star gather, plus a single target NP, he will deal a lot of damage regardless of the gimmick. That does make him inferior to Raikou for the most part, but is still more practical than trying to make low HP a thing without him having Guts built in.
In that vein I’d definitely put him into Tier 3 or 4 instead. At best he will deal insane damage, but his best is really hard to make use of and basically impossible to pull off consistently, so he doesn’t deserve being on the same tier as heavyweights like Drake, Okita, Nero Bride and Cleopatra in my opinion.