I saw this kind of army the 1st time when i faced it, and it crushed me hard. The 2nd time is when i try this army myself and i saw its strength confirm : unstoppable.

Here's the strategy : Endless waves of cheap-halberdiers, sometimes champions rushing forward. They act as meatshields and decoys, while backing by H-Scorpions who act as main damage source. The meatshields do their job well done when luring most of the enemy's damage, and still could maintain the number thanks to mass-reproduction. The H-Scorpions do their part as staying behind to not get attacked and still deliver massing damages to anything in their range.

I have tried myself with this kind of army against Paladins and Huskarls, they beat them hard.

The main reason i realise myself is, While the cost and the attack mostly the same, Paladins always at the frontline, so they always get hit by anything, and will reduce hp eventually (or dramatically - when facing halbs), meanwhile the Scorpions, they could deliver huge damages thanks to mostly staying behind danger and their ability to damaging multi-unit.

I saw an army of 40 Huskarls marching towards 30 Scorpions and they get crushed even before touching any Scorpion.

I can use Paladins to sneak pass the meatshields and go to the Scorps. They do destroy the Scorps, but still loss all of the Paladins afterwards. And that army still be re-reproduced and move forward.

I may think of Siege Onagers. But what if the front is large ? Can you manage to micro every single of SO to shoot at 2-3 Scorps at a time ? Yes if it is not many Scorpions. But against a mass, i think its very hard. Still you lose time to micro SO, while i lose no time since i could use patrol and the Scorps always move forward for me.

I think an army of Korean SOs will beat this army fairly enough, but not always we can be a Korean, while alot of civs could use this strategy : Goths, Vikings, Chinese, Japanese, ect...

Yeb im not a pro yet. I post here in hoping more suggestions from pros.

if he does scorp pike combo u need combo urself.
actually a lot units work but ur micro is important.
can shot them with trebs, canoners from siegeworkshop, onis, rams.
can atack them from the side with cavalary, huskarl eaglewarriors(thos 2 good vs pikes aswell).
if ur micro is good (or ur lucky cause enemy didnt payed attention for 2-3secs) u can kill all scorps with just 2-3onis.

and dont forget to push after u managed to kill enemy stuff.
scorps only effective in big numbers. 10-15 scorps u can kill easy with some cavalary/huskarl/eagles.
if he manages to rebuild ur hunt for the lucky oni shoot starts again.

Onagers. You can cut the trees to chase them with cavarly too.

PD: Rams can distract pikes and champs while onagers shot, and they can destroy HScorps too.

Scorpions... really...

No combination is unbeatable, certainly not this one. And it's not just the army composition, it's the micro and execution.

"No combination is unbeatable". Well, theoretically, its true. But given the wrong condition, some armies are incredibly hard to beat, if not say invincible.
But what do you suggest ? Dont just make theories.

spam huskarls!

Flanking could work, especially as distraction/hurt attacks (2-3 hits then gb).
Use other siege weapons to take out scorpions (canons or orangers)
And I believe towers/castles could stop scorpions+halberdiers...unless he switches to canons/trebs.

scorpions are slow, if they are on open land you can strike directly at scorpions with heavy cavalry or perhaps elite eagles on staggered formation to avoid projectile damage

for maps like BF, i think the best way to counter mass scorpion army is with onagers or cannons (both outrange scorpions and have area damage so they can kill many scorpions in one hit)

Depending on the civ you are, you can do many things.

You can counter with: mass husk (when i say mass, i mean like 90 husk), champs + archs (need micro) , canons + hc (need micro), SO + skirm, full BBT, full mameluk. Well there are a lot of possibilities but you need to micro them well, dont lose army in vain.