Honestly the hitboxes for the enemies are way too small, and the amount of health the enemy has is way too much at the beginning of the game.
The upgrades are also way too expensive, and the what the wizard does is not clear (no pun intended).
At the start of the game is when the player is introduced to the game, so the difficulty should be relatively easier so that players can explore the mechanics of the game, so variables that affect difficulty such as speed of enemy, hp of enemy, hitbox size, upgrade cost, should all be taken into account.
Playtesting the game multiple times is one way to find out if the difficulty scale is too easy or difficult. Also one thing to take into account when playtesting is that usually if you find the game difficulty just right, new players are going to find the game too difficult.