Lots of skink skirmishers. Multiple poison shots will really hurt this guy. They come in boxes of 20 or 24 so you should get plenty of them. If you can shoot him and then get him to charge you while you stand and shoot you're going to get a lot of wounds. Lets say you have 10 guys who rapid fire. That's 20 shots, 3.3 of these should be 6's which skip toughness and now he's relying on his 5+ save. He should save one of these so you'll get 2 wounds. If he charges (from more than 7 inches away) you can stand and shoot. This gives you another 20 shots, 3.3 6's and 2 wounds which should kill him. Not too shabby for a 70 point unit. Does this help?
Tomb Kings player here... My tomb scorpion always gets shot by bolt throwers, which work fairly well against it (although I can generally use this to my advantage because my other units are not being bolted). 235 points for a vanilla steggie is kinda overdoing it used purely to fight an 85 point scorpion, though. Otherwise his weaknesses are high strength attacks or poisoned attacks because of his high toughness. And for Sotek's sake keep your heroes away from him, killing blow + poisoned attacks make him very kill-y!
Hmm what do you normally have trouble with the scorpion doing? I always used my scorpions to hunt warmachines and lone mages, but LM generally don't have either. Static combat res will be a problem for a scorp. Overall a unit of skink skirmishers as Sebulba said is probably the easiest way to deal with them. Just leave the skirmishers near where the tunnel marker is but not close enough to get charged straight away if possible.
A skrox unit of 11/1 could do the job nicely. Two ranks, outnumber, and possibly some wounds from the Kroxigor, who is of course immune to killing blow being US 3.