If you are playing the original version of the defender 2 game, you will notice that your hero can't go beyond level 30 by default.
That's because the developer wanted your power to become stronger gradually instead of being godly from start! You have to gain more XP and level up manually by gaining more exp per win.
However, if you are using Defender 2 mod APk all the levels limitations are removed so nothing prevents you to power up hero way faster! :-)
You should also know that def2 mod apk has an unlimited fireballs option which results in higher scores. More score means more XP and faster level up.
If you look at the top players of defender 2 game, they all use modded apk due to the unlimited fireballs feature that allows them to spam fireballs anytime/anywhere with no drawbacks!
Engage multiple waves of monsters at the same time without any effort by hitting them much faster than the original version.