How to get souls with coins?
If you are just starting the game this post is probably not going to help you, as you won't have that many souls, however, if you've been playing the game for more than a year, all of your support cards must be maxed out, as well as your rare equipment, meaning that there is no place for you to spend all the coins you accumulate every day.
Unfortunately, there is no option in the Mortal Kombat Mobile store which lets you buy souls with your coins. However, there is a way that can allow you to gain some souls with the help of your coins.
A lot of people stop doing challenges, once they max out the character. Well, I don't. I try and complete every challenge on all difficulties, but I only fight on auto and on the last 2 towers of each difficulty. For the rest of the towers, or at least for those matches that can be skipped with coins, I do (skip the matches using coins). That way, completing a challenge on all difficulties can be done in less than 1h, without you even actually playing (as you can use auto) and it can get you more than 500 souls, counting all the re-sells you will do if you have that given challenge character maxed out.