It’s not like Ness is gonna be getting hit every second, he has options to avoid burning through his shields and he can retaliate in a number of ways. The most costly shield Ness can use is only 10 PP. And I think it’s a bit much to assume Mecha is gonna perfectly avoid everything Ness does, especially on first try with stuff like PK Flash/paralysis into rust promoter. I’m pretty sure he can be caught off guard like anyone else. There’s always a good chance that Ness gets off a Flash and wins right there.
And now that I think, Mecha’s explosion manipulation is granted by missiles which usually have to make contact with something to explode. If he’s even a bit off and Ness dodges, I’m not sure that AOE is gonna stop him since it’ll just keep traveling until it hits something way farther away. He only has several meters of range with guns and missiles, so his range spam strat actually works against those points. All he has is energy and teleportation for going that far. And if that’s all, I think Ness can eventually either force him to try going up close and get hit with a bat, rust, or another paralysis, crush him if he tries a spin dash, or just get a PK Flash off that ends up killing Mecha.
All Mecha has on his profile is town level so at best he’s 99/49.5 kilotons, so just a bit over 3x Ness without Defense Spray. I think Ness can manage that with all his options as well as healing and a decent level of regen which is cited to let him
recover from explosions. Mecha doesn’t have any homing attacks, so hitting Ness’ vitals is easier said than done. And I don’t know how Ness is gonna be burning through his items with just how many he has and how much effort Mecha has to go through to deal significant damage to him. I don’t think keeping a shield at all times will be very hard with the low PP cost and recovery items. I’m just gonna assume Mecha’s best is a few hours to maybe a day since he’s massively hypersonic+ and has AOE, so it may not of even taken that long to wipe the planet.