In my experience Arc 3 is when you have to stop just brute forcing things, you have to be more tactical/aware. Interrupts, target priority and equipment start to matter more.
On some characters/gear sets Act 3 is definitely the end of my journey, so it's not just you, but with some adjustments/research you might find it's just the beginning of the challenge.
I’ve made it to the Arc 2 Tho’at with a companion but not defeated it yet, but maybe with another player we could do better, I just haven’t had the time to try yet because of RL.
with oakensorc HA build and isobel healer (just green pieces) i've reached arc 6, but it was tough, especially arc 4 onward (companion get insta-killed at the start of every round)
with a good build and a bit of luck in finding some strong visions, you can reach arc 5 (required for achievement), don't give up OP!