I was caught out a bit when I first played a decent Joker but it's really not as hard as you'd think. Just walk him down if he starts with gun shenanigans and block accordingly. IADG when you can too, he can't do much about it. Don't get too wakeup happy, it's easy for Joker to do setups to avoid it and full combo you. Interrupt blockstrings cancelled into specials with holes in them like most of Joker's strings into low canister/crowbar but be wary of strings cancelled into flower. Uppercut his jump normals as well, WoWo has a great d2. TECH ROLL ANYTHING AND EVERYTHING BY THE WAY, Joker is free to tech roll lol.
Lex is a bit tougher but you just need to hold your ground. Again don't get too wakeup happy as trait will just absorb it, he'll do a combo and end it and get trait back. Just block on wakeup. From mid range MB tiara if he tries to throw something out to break his shield and stop him from getting a setup/combo and allow you to get in. Don't use IADG too much, ESPECIALLY if he has trait up, you are just asking to get comboed. D12 is very useful for breaking his armour but be careful of course.
As for oki artillery strike you just need to deal with it sometimes. Maybe you could backdash it but I doubt it. Tech roll as much as you can.