level of experience?

Lots of long hours on the unit builder and mass battle sim are my excuse.

My latest project has been trying to do something with Barst. I start with the recommended build, analyze where he excels, and try and go from there.

Well, for me it's always a joke in the first place. If I'm just running around talking with a friend who plays, once we run out of IRL stuff to talk about (mostly how college sucks 90% of the damn time) we switch to games or movies and just start yammering about crazy shit, and then if one of us is willing we go ahead and make it a reality just to goof around.

For example, my crazy as +8 P!Olivia was literally a waifu joke for 2 months before I went and said I'd make her as insane and usable as possible and then, well, she's stupid. It's a matter of momentary stupidity mixed with some actual strategy that makes those kinds of units work in the end.

Now, if you want to learn from experience and get good ideas, a good tip is to at first, learn the AI in Arena and how to manipulate it. Go watch Akariss, since I believe he has a video on manupilating arena AI. That should set you up for arena at least. The rest is just getting into the swing of things and giving it a big go.

So, if you make some crazy build, just go ham and post it, ask for advice and maybe we can help grease the creative gears in your head. Cause trust me, building stupid crazy stuff in FEH is probably some of the most fun you'll get (and watching your unit succeed with it)

Think of making builds that are synergistic.Take a little bit of your time to think out how the skills would bounce off each other.
I remembered once when I was a noob I made a build using Fury and Guard.Fury lowers HP,while Guard only activates at 70%+ of HP.I beat myself for wasting a Hinata.Instead I settled with Steady Breath and Aether.It costed me a little bit,but that was worth it.Preventing specials were a good way of keeping you HP,and on the enemy phase with Steady breath,you are guaranteed to get 4 charges (or activate Aether if you use QR or are able to double naturally) unless the enemy,ironically,has Guard.

Experience is a part of it.
A large one really.

Once you kinda have to logic in the back of your head, the builds come naturely.
It's a combo of logic and creativity

What I do for characters I want to invest in is I don't look at any of the builds they recommend and then I do different builds I think that would be good on them in a simulator. A lot of it came from experience with the game and doing stuff on my own. Previous FE games don't really help you come up with builds.

https://arcticsilverfox.com/feh_sim/

I just mash things together and it probably works out but I don't have that unit so I just sit there and wait for it to become a Focus unit to test it out.

Well from my experience, every unit is good. Buuut most of the time it all comes to ivs and skills,I'll use your team for example (it's really cool BTW)
Lilina it's a unit that has been in the game since day 1 so her stats are not that great, but with a +attack (her best stat) -def(her worst stat), her refined tome and deathblow 3 she can be a magic nuke with all that attack.
Nowi it's another good and fun unit and has a lot of room to grow, she can be a magic or physical tank or a player phase unit with the Right set of skills.
Female adrift Corrin it's the most recent unit in your team, personally I don't like new units that much because they already have the best kit posible from the start, so you can't mix and match any other skill without feeling like it's a "waste" but what do I know right?
You can make any unit you want op if you have the resources and the time