How does a particle weapon with a muzzle velocity so slow render lasers useless? Or any decent alternative particle weapon with a reasonable muzzle velocity? Besides, if the muzzle velocity is so slow for a particle weapon, they need to be shooting antimatter for it to do any damage.
And that still doesn't answer my question: Why does one side hit targets every time whereas the other side misses every time? Does only one side in this war have actual trained soldiers piloting their mecha? The other side is using some random dudes from the streets who never in their life piloted a vehicle more complex than a car before they were suddenly sent to fight the enemy?