Formations often depend on what you are doing and what your playstyle is. For example, many people like to run with 2 fury thieves in tower, either 2 single target fury thieves or 1 single target and 1 multitarget, to allow for more of the party members to use their skills. Others prefer to brute force their way through content with event heroes, who can decimate most tower enemies simply with basic attacks. Some like to run 1 dedicated row healer, while others like to run 2 full party lesser healers (axe main/ereina). There are also many other comboes that people like to use.

As far as tower parties go, Ereina usually falls off at the end game, where you may like to supplement her healing with axe main, or another dedicated healer entirely. Karinna fares quite poorly as a tank, so consider replacing her with someone more durable later on. Early game is a good time to experiment with different heroes, so use the time you have to find out what works best for you. Just remember, in the end, you need 9 heroes for tower and 15 heroes for hero wars.

Thanks! I've never even considered changing my main, I might do that later in the game if I start doing poorly with my Gladiator. I took Vera out of the formation, since I already have plenty of 3x3 AoE with my Gladiator, Karinna, and Ariesse, so my new formation is Gladiator and Karinna in front, Ereina and Ariesse behind them, and Nicki and Black Pearl in the back. It's working pretty well so far, I've boosted Nicki's initiative to highest since she gets fury back with her skill, and Ariesse is lower initiative since she isn't as high level as Gladiator or Karinna yet. I'm working on leveling up Robin and Khalid for use in hero wars, but honestly right now my main focus is the main storyline since hero wars seems confusing with that many people, and I've made it about as far as I can in the tower for now.

I would recommend replacing Karinna back with Vera for the following reasons:

At lv.50, Vera has higher stats, meaning she has better tankability. Being 3*, she also has better promotion potential (she can gain more stats just because she can be promoted more times). Defense debuffs, when use early in the turn, can also help increase your party's damage output.