Didn't watch replay.

Scorpion has amazing DPS, so it's like a riot in the sense of mace/redback, even shareing redback's weapon identity with LPBs. It can get overwhelmed with raiders, but if you send too few they will get DPS'ed to death. You need to approach carefully, and if you do colse the distance with a mass of raiders, try using scorpion's turnrate and arc of fire toyour advantage, splitting your raiders to chase around both ways and keep as many behind out of range of the red lazers as possible.

With shield lab you can use a couple racketeers, 4 or 5, to disable it quicklt. if afraid of getting ambushed, have some bandits screening. if you don't have time, use a couple thugs to screen ahead of the main shieldball. losing a thug or 4 to uncloak and disable the scorpion is better than balling up and letting everything get stunned.

It costs 800 metal to construct new tech. Bandit is a weaker raider, with lower DPS and speed than others. Fleas or glaives can close the distance faster. Besides teching to just raiders, you can tech for other anti heavies, like widow or ultimatum. I like teching into striderhub/athena, which does cost 1400 metal, but athena can move quickly and build many useful units near the front lines, like fleasa, bandits, widows, cloakers/shields, some sneaky djinn play etc. and these units can be built remotely on the map.

Fac-swithcing in responce to a scorpion is viable on a map like otago, where half the map gives you like 35 m/s. If you are responding quickly to something unexpected, you need to focus your build power by setting things to high priority. 2,000 metal a minute can get a couple racketeers and screening bandits up, or fac switch to spider to get widow, or even athena path. Stop focusing on light defense and push your economy all in on a sufficient countermeasure. It takes 2 widows to stun a full HP scorpion, and it will only stun it for 14 or so seconds, so investing in a third is a good idea to keep it stuned. 3 widows drains a fusion's worth of power, so having excess energy to do this is a must.

Ultimatum is ofcourse a catch-all if you can get the drop on any unit. it's very slow, so you should be able to think ahead to use it/ bait the enemy strider in a direction especially a cloaked unit like scorpion.

Don't sacrifice your army if a countermeasure will arrive soon. If the enemy has riots, send raiders elsewhere, even idle in base, untill they can be used efficiently again. If scorpion wrecks your house, tactical retreat and use the strongest units together with everything you have simultaneously to take it down.