Ever since the 4.4 update, coin farming is easier than ever!
However, many updates later, the Pixel Gun Company has patched coin farming, making coins harder to obtain. Don't worry, there are multiple methods how to earn easy coins! In addition, if you want to level up, this tutorial has methods too!
You can do the same method, but everytime a match finishes, leave the server and go to a different server, as you will usually have less time to fight during matches. Be sure there aren't any tryhards or players who have scores that you can reach in different servers.
You can do the same method, but everyday, you can earn double the rewards in all modes for the first time you play in a day. It's suggested to play all modes at least once, if you want to earn extra coins and experience.
Also, in Campaign, if you have a 2-stars level, go to the easiest level in which you have 2-stars, equip your best loadout and kill monsters, but unequip your armor. Then, note that you should take health damage, otherwise, you will earn 3 and 5 , and won't be able to earn coins from that level again. Note that this method only works if you want to farm coins only.
Alternatively, you can complete Campaign by obtaining as many rewards as possible from there.