well almost never

I feel compelled to write this thread becuase I'm not that good but I find it easy to get top score and top kills with a long tom. At the same time basically never getting hit by a tom tom when changing into a mech.
For new guys you should know there are 3 radar modes.
...o,O
YES 3
Active Radar
Passive Radar
Ping Radar Mode
So what the hell is Ping mode ? Its really simple go to passive radar, and when your start getting close to enemy positions double tap the r key "PING" see a red icon well thats your enemy , guess what even in a fast mech even though your radar can't see him odds are he's close to where the ping put him.
In a long range unit find some high ground and look into the diretion you got the hit, you may be able to get a visual. Once you've got a visual guess what, you don't need your radar so leave it off.
(exception to this rule is LRM's you need active to lock, BUT you can fire in passive mode with a tag or narc active so keep that in mind)
if your close range setup then just approch the ping and ping again when your closer if you can't get a visual remeber every time you ping your enemy will see you as well.
Why is this important ? Long Toms have hard time ranging you out in the time you flick your radar on and off. The first time you do it and 4 seconds later you see a long tom shell hit 50m away you'll smile

So is that it ? immune from long toms ? not quite

If an enemy gets you on their radar screen even if your in passive mode you will pop up on the long toms radar screen, however if your outside the radar range of the long tom then he'll get spurts of targetting data dpending on how close the enemy keeps contact with you. While this is harder to range find its not impossible so be wary passive is not some invunerbility shield.
The BA scout can use the above knowlege to help long toms catch people in passive mode. Better yet if you walk and don't shoot the mech your shadowing he won't even know he's about to get slammed by a LT shell.
But at the moment with so many people still using active radar , most long toms will ignore passive targets in favour of more easily hit active ones.
Now for those who still use long toms and I include myself here beucase long tomming can be fun. You still need to use passive radar in a long tom as well. if your travelling to a location to fire from why is your radar on ? it should be off , if your not shooting your radar is passive.
If while shooting you come across a long tom on your radar go passive and MOVE ! sure he may not have seen you but chance are by the time you've seen him he's already lining up a shot on you. Now that you've moved ping radar and move again now you know the approx range and loc line a shot up ping again and quickly fine tune and shoot or just shoot without a final ping.
LT v LT battles can be alot of fun , one time I exchanged about 5 shots with an enemy long tom watching shells hit the ground where I was moments before he finally got me on the 6th shot but was a great duel.
Just a little Story to finish on
Last night I walked an UAC20 Mauler the length of the sandblasted map on the oppsite side of the map that the main battle was taking place. our guys were getting killed by long toms.
I left my radar passive for the whole trip didn't even do radar pings. just aimed for the spot where most mech exit the enemy main base and low and behold a long tom which I proceed to walk up to having not been noticed by anyone.
First he knew I was there was the 2 UAC20 rounds that slammed into him although being on the ball he did eject moment before I totalled his LT. And by a spot of luck I caught sight of a second long tom which I proceeded to "Maul" as well

becuase once your under 200-300m range on an LT they have no chance of hitting you. (unless they're setup on a hill and pitched forward so they can hit closer targets)
There is no specific mech you "need" to use to counter Long toms , its all about managing your radar.