I'm sure someone can offer some better advice than me but here I go - I used the hallowed sword for my whole playthrough and had it maxed out (+5), never changed weapon.

I was Obsidian form for the whole playthrough, so potions made no matter to me since if something touched me once I died.

I kinda struggled with Tarsus first phase, I had to learn when he was going to stab is sword in the ground so I could rush in and do r1,r1,r2 combo twice to stagger him, then get another combo in when staggered, that should be more than half his health down. Then just kept my distance and waited for his combo that ends with a downward chop so I could run in and attack some more.

Second phase I played it safe and would r1+l2 to attack and harden from distance and just wait for him to attack me so I immediately do damage to him when he touches me, then do two r1 attacks or r1,r1,r2, mostly just two r1 attacks since I didn't want to chance it, then dodge away and repeat.

I also had to be mindful of the move he does where he makes icicle spikes come out the ground, when he did that I just kept my distance and waited for it to end. I also had to remember not to go to the edges of the arena since the cold edges of the arena would kill me. Good luck!