Assassin and Caster 3star

Hassan of cursed arm for assassin and Hans for caster. They may be two stars but are your best bets. Hassan can create a good amount of stars and Hans is a budget version of Waver with great team support. Both are easy to NP5 and are useful to even 5* rich teams. Definitely focus on those two.

Cu caster and any of the other Hassan’s would be my other recommendations for caster and assassin respectively. Cu has good survivability and Hassan of 100 Persona can do decent damage with her crits and NP

For casters, medea by far. Np spam with some utility. Or babbage. He has a buster deck, which is weird for a caster, but it might feel a bit more familiar if you mainly use saber or archer classes. If you are looking for a support, go with hans. He is a staple even in the late game.
For assassins, fuuma. Good stargen and a targetable evade. Otherwise hassan of cursed arm for similar stargen and amazing staying power. For boss killing, jing ke can do very good damage with her crits and np, but sasaki kojirou might be easier to use and raise with an emergency evade. If you want arts, use hf hassan for similar effect.
Keep in mind the PAID guaranteed ssr banner at thr end of the year has the best servants featured in the caster and assassin banners. Also, make sure tp get np5 for whatever servant you choose. Try playing with a friend support to see whivh ones you like.

Most Caster roles are to dedicate their power to support your team, but few of them
were an exception like Medea, Geronimo, Mephistopheles or etc.
From the 3 star Caster, the good ST Caster is Medea and the good AOE one is Chu Caster. But Chu is story locked so i think the best easy to get AOE 3 star Caster is Paracelsus and Charles Babbage.
Charles Babbage is a Caster who want to be a Saber so he hits little harder than most of his 3 star Caster rival.
For faster farming and more utility Paracelsus is worth for the investment, his NP is Art but it quite pack the punch.
For me, i choose Paracelsus for the AOE Caster and Medea for the ST Caster.
But if you get Chu Caster NP 3 ++, he's more valuable than the rest but it still up to us as their pilot.

For the Assassin, Kotaro Fuuma is the best available AOE Silver assassin for now.
Hassan of the Hundred Persona is good for your main ST assassin for rider boss killer.
For the unique one it's Henry Jekyll & Hyde. Some veteran and seasoned player alike often use him as their main assassin, i would recommend Henry Jekyll & Hyde since you need to be a little careful while using him and if you can use him, he is pretty strong for a Crit Zerker and mostly people use him as a Crit based assassin/berserker.
Can be swapped to Zerker class or depend to the stage conditions ( once he change, be can't swap his class back ). He has double class, an assassin and a berserker.

Offence or support caster? For offense the best is Medea while for support there are quite few caster available such as Hans(multi purpose support), Paracelsus(art support), Mozart(art support, debuff NP), Shakespeare (Targetable NP charge, Buster support)

I was in that boat for a while starting out so I leveled quite a few of the low tier assassin and casters. The ones I liked best and will still use are von hohenheim and single grailed kojirou. Medea is still a good one, though my fancy new da Vinci can hit as hard with aoe...so I don't use as much as von hohenheim. I do prefer medea and von hohenheim to cu caster and Babbage because they are much easier to np spam.

The novelty of jekyll's class change has worn off and haven't used him in months. And I think kojirou is just more fun than jing ke so I still pull him out when Jack needs help. I like fuuma too, though don't use as much now that I have scathach assassin, though I prefer fumma's animation to scathach.