5
Right, I'm going to have a quick go at answering the original question (what options, other than arty, are effective against a heavy hill camper) rather than wading into the how to use arty discussion that this seems to have become.
The answer, as always, is - it depends on what you want to do, how you want to achieve it, and what else is going on. I'd say you can split the options into 3 categories of choices. Assuming you have an enemy with good long range firepower in an elevated and covered position you can.
1 - mitigate it. Your opponent has invested significantly in this "sniper", if you can stay out of it's way you can achieve local superiority elsewhere. Break lines of sight, offer only poor shots, add smoke, make the conflict happen where the sniper cannot see it/significantly affect it. This expensive investment now has less affect on the encounter than it's cost justifies and it's commander must either accept this loss of effectiveness or surrender the high ground to move to a less defensive position where they can affect the encounter.
2 - Assault it. A long range mech on a hill can be efficiently overpowered by short range mechs on the same hill. Using good counters is especially effective. For example an AWS-8Q isolated on a hill on overwatch is in trouble if a fast VTOL squadron drops 8 platoons of jump infantry on the same hill. Those PPCs suck against PBI and while the armour will hold up a while the AWS will get quickly ripped to shreds by close range infantry fire, especially if it refuses to move.
3 - out shoot it. Take advantage of it's limitations to get advantages in a ranged firefight, it has ERLL, you use LRMs at range 21, it has PPCs, you wheel some field guns into range and let them kill tiny numbers of infantry while hammering them with autocannons, this is where Arty comes in, or Ortillery, or bombs or IDF LRMs, or using C3 snipers with a hard to hit spotter, or fast moving VTOLS plinking at long range that make them impossible to hit. Are they using LRMs? Offer them terrible target numbers and AMS and watch them empty their ammo bins for almost no damage or refuse to fire while you roll 11+s with energy weapons in return.
If they have fixed on a strategy, turn it to your advantage. If this gives you the opportunity to divide and conquer, take it. If you can mitigate enemy assets for minimal cost, do it. If your opposition becomes static, rearrange your forces to offer efficient counters. Potentially there is opportunity to play the objective without having to engage some enemies, if so, achieve the objective and withdraw.