One good place to get good growth are cards from raids and patrols. Learning to maximize against them is time, patience and willingness to learn and try different things. These select gear next are low level gear options easily gotten from the game. Some others work better for patrols but these mentioned below are all useful options for Raids.
Astral ring , mystic jade bangle, iron cuffs, metal rockers, chain arm, spirit touch, air heroes, druidic headdress, and angelic band. All these options are available in store and help you in other places also like missions and patrols possibly even events for swap gear. The point behind using these options are more towards raids since these gear options offer some kind of damage on them vs resistances. These options open up lower than any other options usable for raids but I've also found a few more in patrols and missions that are a nice addition to the swap gear for raids like Frankie's arms there are 3 different ones same damages just upgraded a bit on each. For patrols and missions when these fail you there is other options but will recap on these at another time.
Another place to get MP gear for raid is daily log in rewards the 50 day challenge. In the 50 days you get a lot of experience but also get MPs , Cards , a ring , and on day 50 a core. So if you log in every day for 300 days and do the log in challenge collection you will have a core of every type a ring from store going with each core type so physical core physical ring and so on down the rest of the new core vs store ring each time you run the 50 day challenge. It doesn't matter if you reset the clock after you collect or not the next day you will still start over on a new 50 day check in challenge. If you don't mind leveling up some to get the rewards I think it can be worth it for any free or budget player to gain access to these items for free.
Collecting gear for later is essential just like gathering cards. Without some effort put into the learning curve you will remain at a certain point without growth going forward. Challenge yourself to try to beat city bosses or patrols with these new explored options. Go revisit the Raids study up and bring a new set of gear into the field capitalizing on damage to raids or survive a boss in missions you didn't think you could.
Once you start playing around with gear you will just want more and more gear to swap out as you start to figure out ok this gear here this gear for that and so on. Get that concept down and gear swap will be a new regular part of your day too.
If you play to be the best at your level these options might not even be useful to you. To everyone else who is playing to try and better their toon learning to swap gear around is a big part of the game. Patrols and missions: as you are progressing you will hit fights you lose. If you watch the fights, the ones you lose to are the first line of swap gear opportunities. Look at what damages you are doing to them and them you. Go back to My Hero page and look at your core and your damages say you have on the technology core , if you hit them for say 0 technology and 30 supernatural but you have 40 supernatural then you know they resist 10 damage and all of your technology damage. They hit you for 10 physical and 15 technology my first thought would be to swap core to supernatural to stop their damage, next I would remove any technology damage gear and replace it before trying them again. This way you can continue to push farther into patrols and missions to gain rewards.
It's all perspective when it comes to well when should I start patrols, missions and raids. It all depends on how you want to play. Me I have hit every raid some for very little to others over 200k damage. There is no right or wrong way to play, it's a game have some fun with it. Raids missions and patrols are there to feed people experience cards and gear. So really depends on where you want to be level wise as well. If you want to stay put I wouldn't advise joining these since you are constantly going to gain experience with them. If you don't mind and just want growth then beat it up. Hopefully this helps. Anyone want an article about something in specific let me know I'll cover what I know about it.