I'm creating this guide to help everyone because playdumb has created a game that makes people feel that they need help with this game.
I'm currently lvl 88 with a pvp rating of 1288. I may not be the best player out there and am a free gold player, but I am willing to give tips.
Your first 90 char. buy should be between two people CAPTAIN AMERICA or SPIDEY. Cap A has great skillset (credit goes to Namikaze116 in the post below) which makes him do whatever you want. He is better suited for pvp esp. when u unlock his WWII suit. Spiderman on the other hand UNLOCKS epic boss 3-5 (doombringer) and 7-3 (goblin glider, which is a GREAT WEAPON that is USEFUL throughout all game) and premium 3-6 (which the reward for GSE is great esp. if you're lower level).
The choice is yours whether you want better weapons (Pick spiderman) or a great hero (Cap A).
Often times there are opportunities to earn free gold. On the screen to buy gold, click earn gold and go to the category of free. Most of the time, there are surveys which take a while but hey you get gold.
You could always abort a mission after getting the roulette if you don't want the mission to get harder and would rather have a breeze in trying farm weapons. ONLY DRAWBACK are not GETTING THE 5 CP mastery, the chapter reward, or the lack of exp. It also makes me feel better about myself knowing that I got a weapon and I just 3 starred it (even though I've done the mission 20 times already).
If you want to get command points, do mission 1-1. Only need 3 hours of energy and you are able to go to the roulette for a chance of 1, 3, or 5 CP. Later on you can go to 7-3 for better rewards.
Don't bother researching Isos. Only research them when you are level 65 and have unlocked the shards. There's no research requirement for the shards if i remember correctly.
PLAN when you are about to level up, so you don't waste energy that you could have used to do one more fight.
Don't be discouraged when doing PVP. Many times I do PVP just to challenge myself and see what others have to make my team even stronger against the AI. If I lose, I try to see what makes their team so strong and analyze why I lost. It could be because they got a good weapon or a hero that you don't have (meaning you get that hero next too). True, it's not fun if you see people have RIDICULOUS drops (like 40 MM) or gold items (36 QJs) in the armory but know that PVP rating doesn't matter, you're just making sure that you have a strong team around your level. I feel that doing PVP makes you realize that whether you have a good team for your level. Plus, you could get drops that you can sell later on.
Finally, the MOST IMPORTANT RULE OF THEM ALL, ALWAYS READ THE FORUMS. The forums always give good advice and would show you other stuff. I see so many people complain/post on the forums that they could not have gotten emma frost because they didn't have Colossus, SW, or PH when the guide was posted (on the forums) in the beginning of Spec Ops 2 saying you needed those characters. CP drops is one thing, but not knowing the requirement is even stupider.
Thanks for reading all these tips.
Veterans feel free to post others that I might have forgotten to mention.