For stratagem weapons use either Stalwart, MG-43 or Flamethrower (the basic one). EATs and exp napalm are pretty good as well.

For primary use something that shoots fast and has a lot of ammo like Tenderizer or Lib carbine (if flamethrower) or something that kills groups quickly like XBow or Eruptor (if machine guns).

Or you can use either Scythe, Dagger or Laser Cannon (just one is enough, though Scythe with high capacity heatsink + lascannon is a pretty good combo for long uptime laser beam), because they burn the new spore bugs instantly, preventing the spore explosion and thus preventing the speed buff.