Since release I saw countless threads and comments over people complaining that they hit a wall and can't progress anymore. Apart from switching out minis to counter the mission, there will be a point where the game wants you to either spend money to fasten up your progress or literally just sit out some time and come back later.
These types of games are supposed to start out easy with many rewards and upgrades to the point where you will be reward starved outside from your daily and weekly rewards.
To help and maximize your progression, heres an overview of content you should do daily/weekly outside of the campaign.
Quests - Good source of experience, the daily quest limit is 20, but they will stack up to 300 if you miss some.
Arclight Surges - Your main gold income, resets every Thursday and Sunday
Dungeon - Weekly faction dungeon which can be repeated countless times, but it gets harder and harder
PVP (optional) - Not for everyone but a good source of experience
Free shop claims - Free offer every 4 hours, mostly experience for random minis, sometimes rare rewards
I hope you are enjoying the game and have fun and remember this game is supposed to take a while and your progression will slown down eventually.