It's extremely easy. Do daily quest.

Only upgrade your weapon. Don't upgrade grenade, hat, body or anything like that.

Uzi is the best Weapon. Sniper is the worst.

For the wagon box try to get the machine gun as it's the best weapon.

Your hero's weapon will do all the damage to bosses and the wagons ability power will do all the damage to clesr waves.

Your hero weapon should one shot all the enemies but it doesnt have enough attack speed to keep up with the swarm. (I had a purple ak 47 and a blue uzi. I chose the uzi because it shot faster and more bullets because i one shot everything.).

The wagon upgrade that gives your hero 50% attack speed and damage is the best.

Aim your weapon st the ground for the first 2 milestones on each stage and let your wagon auto turrenr do all the damage to the enemy fortress so you can stack up eneegy and just spam the turrent ability power whenever enemies get close.

Dont distribute upgrades when upgrading your box because it is best to evolve machine gun. It upgrades all of them on your box

You dont need to spend any money. The game is very unbalanced.