As someone who regularly spanks Theseus and the Bull (seriously I think he's maybe beaten me like 3-4 times total), my standard strategy, regardless of build is as follows:
1. Focus on the bull first. He's generally the easier target of the 2, and when he gets to 50% nothing extra is added to the fight to make it harder, like it does with Theseus.
The bull will do 3 of those leaping axe strikes, always aiming for your current location, so just dash around until he's done his 3 reps. Go in for some hits, and avoid his regular axe swings. Keep an eye on Zagreus' body, as if you see that bullseye appear, Theseus is about to throw his spear at you. Either hide behind a pillar (usually a good strategy), or just time your dash to avoid it. It will change color right before he throws it, and there will be an audio cue as well, to indicate the spear is incoming NOW. Hiding behind a pillar usually works, though it can sometimes be a risk if the spear throw times up with one of Astereus' leaping sessions. I preferred just dodging, as you can also do that while dealing with Astereus, but that does take practice to get down the timing.
2. Once you get the bull down to...I think it's 25-30% (maybe 50%? it's been a while since I played Hades so the %'s are fuzzy), he will toss up a shield and ignore all damage. So either use any big attacks, like a god Call ability, early in the fight with him, or just save them until AFTER he's popped his pally bubble. I personally suggest saving the big punch ability for Theseus after he hits 50%.
3. Theseus fight. Ok so, he's only really weak from behind, and he's pretty good at making sure to face you a lot. The one time he always turns his back to you, is after a spear throw, in phase 1 (when he's 50%+health). So just sort of avoid him until he's getting ready to throw his spear, and position yourself so you can both avoid the attack, but also rush in quickly after the spear, to get some hits in. Because he will turn and walk away from the direction he threw the spear for a few seconds. This is your primary damage time for him. Dash into position, gets some burst damage in, and dash away. He often follows up the walking off with an AOE spear twirl around him, so you don't want to be there for that. Just wash, rinse repeat. If you are built for a lot of dashing, and close combat (like with my personal favorite the punch gloves), you can potentially do the fight just dash around him near constantly, getting back punches in close range, and dash out when he does the twirl attack. It's high risk though, as if you don't have good defense or timing, you are right up in his face, and open for damage. Otherwise, just hang back, wait for that spear throw opening, and get your damage in until 50%
4. Save your big hit for Theseus. As stated above, Theseus will make the fight MUCH harder once you get him to 50% health. He will pop a quick pally bubble, call down a random god to help (it's always one you DON'T have, so keep that in mind), and then you have lots of AOE to deal with. It's widely considered (and I agree) to save your own Call power for this point, as it's not TOO hard to get him down to 50% with regular strategy above. Once he hits 50% though, things are not as fun, and your risk of damage is much higher. So, wait until after he's done his Call attack, so that his pally bubble is gone, and follow it immediately with your own to try and burn him down quickly. The less time you spend with Theseus at less than 50%, the better. Depending on your build, and how strong your Call is, you could potentially one shot him at that point. If it doesn't kill him, it will seriously wound him, and then just continue the pattern of step 3 above. If I recall correctly, he will add in a spear lunging dash into his mix, but will still have a short opening behind his back to do some damage.
One thing about when Theseus does his Call, is for the several seconds of activation of it, he just stands there with his spear up to the sky, and shield down. He's completely vulnerable from every angle (as I recall). So if you have some type of attack that you can either fire and forget and it effects an area while you run around, or a powerful, quick strike ability (like a strong bow shot or something), this is a nice time to get some extra damage in on him. The problem is that this is when the various circles of godly damage will be showing up, so you might not have a good opening, but it's a possible time for some free hits, so keep it in mind if the moment presents itself. And to clarify, as I re-read this and felt it was unclear. He pops the bubble BEFORE he calls down the god. Once he's standing there, calling their name, and just standing still with his spear to the sky, he is no longer invincible and can be hit. So be careful not to use your Call ability too soon at this point, or you might have it absorbed by his bubble.
That's mostly it really, for how to deal with T&B, in my opinion. That's what I did almost every fight, regardless of build,boons,difficulty, and it's pretty much worked every time. I'm not claiming to be some Hades expert, far from it, but that fight, I know that fight pretty well.
Hope that helps.
