In my opinion, Rocket Mortars should be removed entirely. Nerfing them or adding a roof over the spawn won't solve their fundamental problem: Rocket Mortars can be fired from cover and strike almost anywhere on the map with zero skill.

It's highly unlikely that Plarium would want to remove artillery aspect of Mech Arena entirely, so I have a suggestion: replace the Rocket Mortars with an artillery cannon, like the Magnum turret from the Tanki Online game.

In Tanki Online, the Magnum turret is similar to a howitzer gun. In the game, the Magnum requires a lot of skill and experience to use effectively. The gun can rotate vertically up or down 90 degrees and fire from 0% to 100% power. By combining the gun elevation and firing power, you can fire a round to a specific distance and at a specific trajectory to fire over walls and buildings https://9apps.ooo/.

The Magnum cannot auto-aim like the Rocket Mortar, and you can add additional handicapps like prevent the mech from moving or rotating while in the firing process.