Cleared up to 39 now

It's after passing level 30, but I can clear several stages surprisingly quickly. As for what to do, once you clear level 30, the Officer Academy is unlocked again, so level up as much as you can afford. For gems, if you have the pink one that narrows the shotgun's range and increases its power, your firepower will increase dramatically. For soldiers, Shotgun, Machine Gunner, Molotov, Priest, Freeze, and Tank provide stability. For skills, prioritize Shotgun, then pick whatever seems strong, like infinite ammo for the Machine Gunner, but for Molotov, I feel like the one that causes wildfire deals the most damage.