basically you want your nukers or damage dealers dealing alot of damage at 100% crit rate and alot of crit damage 200%+, you also want the main stat of the nuker to be high whether its ATK, DEF.....

for supports mostly ACC , DEF% and HP%... you want them fast and providing a ton of buffs... i usually focus alot on slef crafted sets, especially when i started, perception sets allow you to get ahead in ACC so you can land those debuffs, same thing with Resilience sets, easy way to keep your champs alive,, you want to avoid flat sat pieces like this one:

on hands , chest , and feet...flat ATK, HP, DEF isnt recommended, also try to get spd boots for each of your champs...

if you can hit the milestones of each champ in your party then you can manage a lot of the content in the game

even if the gear is 4 star but has the sets and sub-stats you need level it to level 12.

also try to get your support champs with good defence and good speed so they can stay alive.. you dont want non damage dealer champs with attack stats and low accuracy and speed... if your  healer or medic keeps dying how will the team function?

i

Basically - farm the best stage you can, upgrade the correct gear, repeat. There's no shortcut other than spending money, which can get expensive fast. You don't need or want to get past dragon 20. That's the correct stage to be farming. And I wasn't even joking when I said "do that for another year". This game rewards patience.

I tweaked a couple of pieces which cost a lot of silver but I was able to beat dragon 20!

Now that I have that back I'll start farming it for better gear. Yes I need to remind myself this is a long term game, not a quick one.

thanks for the tips!

If you don't mind someone mining your game data you can use Hell Hades raid optimizer to optimize your gear.... somewhat. You still need to understand the fundamentals around your stats. 

I tried it for the first time just recently, but like you said, I need to  understand the fundamentals so when I thought i was doing the parameters ok I realized the next day I was wrong. But, once i get the hang of it I'll enjoy using it.

forge 

Well you are working in the right direction. It takes time and trial to start getting the hang of it and I am still finding issues with my setups and areas that I need to improve when the right gear comes along. 

yes I've been doing that too but I don't have any for 5/6* gear...