not the leig, the leih big arty. the one they build at their base and spam your base with it
If he builds lefh in a 1v1, then you pretty much just won. Your opponent won't have enough manpower for infantry so just steamroll him out of the map, you don't even have to kill that unit.
If you rally want to kill it though, then I have a few suggestions. First, pretty much all late game off map abilities kill it in one go. For usf that is for example pathfinder arty, mechanised arty and armour arty. Just a few munitions and it is as dead as it gets. If you want to take it down even cheaper, flank your opponent with a tank and take a few AP shots at it with attack order on the gun itself. The lefh can be destroyed without killing the crew first and it only takes a few shots to do that.
Btw. It is impossible to build that gun in base. It is like that so you can kill easier, especially as offmaps are banned in base sector as well.