Explanation:

Arachne Queen is a crappy boss that has total defense versus physical, magical, psychic attacks. She also has a lot of add mobs. Thats why players struggle beating her.

Throw Coins is considered an escape move with the flavor text saying throw coins at the enemy so you can escape. The damage itself is just a side-effect of the action, before the escape move happens. Damage is based on the amount of coins sacrificed.

Since Arachne Queen is a boss arena battle, escape moves like Throw Coins will have the escape function disabled. This means players can continuously repeat the Throw Coins animation (throwing coins before running to escape) but since the part where the run and escape action isn executed, you get a move that just causes damage and is not considered to be an attack.

When Arachne Queens HP falls below 50 percent, she transforms. This can be interrupted by timing the damage to cause stun. MC did this and cancelled Arachne Queens transformation here.

With enough coins, and helpers to keep the infinite mobs away from the player, the boss can be beat with just repetitive Throw Coin moves.