Simple concept, not a simple grind. Game Mode Level means Combat Trial Level. Meaning you have to do PVP in order to rank up your game mode level, and unfortunately this is the only way. Game is really unfair at this point, only people who play now are veterans and dedicated players. They will not have any mercy on you. You need 5 points to level up, meaning you need 500 to get to your destination. There are three ways to earn these points in game: Participate, Performance, and Victory.
Participate - Win or lose you get a point.
Performance - During a 1v1, the winner is the player who defeats the other 3 times. If you win at least 2 of the 3 needed you'll get a point.
Victory - Defeat your opponent 3 times.
This means you can earn 1 to 3 points every match. Depending on your skills, you either have to fight 500 times or win 167 times to get to level 100. That isnt the max level but it really isnt necessary to go further than that. This grind will help you get at least 5 achievements though which is nice. You'll end up defeating 100 enemies, you'll be a disciple at you school (only if you have equipment from your school on you as you fight), you'll be able to open your own school when you are level 50, you'll get through Game Mode Level 5 and 100. Also, you need to be at least level 60 to fight all the bosses again twice to get another achievement.
Now here is the thing you might ask, how can you get this achievement? Two ways: Get lucky and end up in a match with a friend or someone willing to help you if you message them (many will not, they will instead offer to train you in the game. It's up to you to decide whether that is worth it or not.) or push through it the best you can. Rematch even if you lose, not many people are on PVP so you'll probably end up fighting the same person over and over anyway. If you find someone willing to boost, all they have to do is jump off the arena 3 times if you are on a good map. If they can do that, the match takes less than a minute and the load up to the next match is just as long so you can grind maybe at least 40 wins per hour, 3 points for each and your friend gets 40 points. Meaning you are basically level 24 and your friend is level 8. Take turns of course. This is really good progress, it took me 6 hours worth of real fighting to get to level 25. I'd recommend doing this all in one go if possible because its luck based for anyone to get in a match with a helpful person.
Best of luck. If you need help or have questions, message me.