Have you ever had a session in which PCs fought against the tank or sth similar? I'm thinking that I could try to throw Leman Russ against my PCs but I need some ideas on how to balance the encounter. Im thinking lots of cover for infantry, some anti armor weapons, lots of hiding places...

It really, really depends on the situation. Big clear field? Infantry is dead. Cityscape and a lot of anti-tank weapons? Tanks are dead, until they have very good infantry support. And all variants in between.

Are you sure Jargal?

It seems to me that even with cover and all, infantry will have a very hard time killing a Leman Russ. Even with a lascannon it takes several shots to kill an LR, and in the meantime, the LR can seriously **** up the infantry or at least suppress them, greatly diminishing their effectiveness.

Leman Russ has very limitad elevation of it armament - and my players was very creative to not fall in tank arcs of fire. Lascannons also tends to deal a lot of RF criticals with their five dices. Smart use of explosive (melta-bombs especially) is very dangerous to armor.

Alright, so you can attack it from a high angle and take it out unopposed, as long as you can deal with any infantry around and the pintle gunner.
That seems reasonable.

RF criticals need to be somewhat lucky though, as not all of them will protect you from the Leman Russ' counterattack, though they do help infantry.
Melta bombs, well, if you can get close to the LR's rear, awesome! If not, hope you get a good RF crit or that I read the Flame quality right and the Melta Bomb will always set the tank on fire, because the attack cannot be evaded.

In which case any Melta Bomb attack on a tank is utterly devastating. Cool.

Don't forget that you can add a trigger to explosive with Tech-Use/Demolition, so you don't need to go too close to the tank - you can use remote activation. The real quest is to lure this tank to the bomb.

Ok, I think I'm gonna make it looted Leman Russ and give it to the orks to drive. They'll turn Battle Cannon into Kannon (the same as mounted atop of Squiggoth Howdah, main difference is that it shoots once for two turns), because the latter does moar dakka. Besides it needs some weak spots. One of the sponsons will be gone - there'll be a nice minigame to try to remain on the left side of the tank at all times. The tank should receive Ramshackle trait (1d10 Critical Damage roll instead of 1d5 when RF). Or maybe not... No, it doesn't make the fight easier, it makes it more random.

Also the battle arena will be stacked with walls to hide for PC's. They'll give them AR, but will perish when shot at, forcing them to change position quickly.

But there could also be one display of power, one shot from da Kannon, that should tell players that one shot can be all that is needed to kill them. Any ideas?

Might've already been covered above, but you need to know how much of a military battle simulator the encounter is. Is it the only tank in the vicinity? Does the LR have vehicle support? Infantry support? Side sponsons? Is the player squad trying to solo the thing, or do they have several NPC filler squads of other infantry, backing them up? Did they come to kill it, intentionally, or stumble upon it, during another job? Once it needs to stand and fight, can the tank turn toward you, and put its rear armor toward a wall, denying you access to its softest armor? A main battle tank is no small, helpless victim, in the annals of 40K, and it takes a special kind of infantry, sometimes, to drop it. If they have specific talents for the job, like Tank Hunters, stealth, and/or anti-armor weaponry, such as the aforementioned lascannon, or krak missiles, though these can suffer from limited mobility, and/or limited ammunition, respectively, it's doable. If the PCs have the element of surprise, and a good, clear shot, they might be able to put two good shots into the back armor, and the Leman Russ will certainly feel it. It'll take their assets a little while to figure out where the attacks came from, apart from "aft and port", so if you can be mobile, you might avoid some retaliation, for a bit (in my mind, this is where the krak missile launcher comes into its own, over the ponderous, need to set up lascannon). Defending infantry weapons can be mitigated by cover, while the tank tries for a firing solution, and what sponsons it is sporting, if any, can matter. If it is lightly escorted, someone did something wrong you might even run up, avoiding most of its firepower, and strap a melta charge to it, or mount the tank exterior, and engage the crew within, maybe with guns, maybe with a grenade, or two, in a very confined space.

If you have ample planning time, and the right resources, you might plant one of those det charges in the road, and ping the tank's belly, to soften it up for your initial attacks. So much of it depends on what you knew, what you know now, and how prepared you can be. If you were a typical Infantry squad, and happened upon a LR, by chance, I'd say "you're toast if you try to engage", and suggest they note its location, for a later anti-armor attack, which they might even later take part in. If it looks damaged, limping home after a battle, and maybe bereft of its pop-gun escort, you might give it a shot. If you have foreknowledge of it, and the right gear, terrain, and such, it shouldn't be too hard, but these are the tanks the Imperium does win wars with, so I'm not sure it'll ever be side-slapping easy.

Well, those are my two thrones on it. Best of luck in cracking the tank.

But there could also be one display of power, one shot from da Kannon, that should tell players that one shot can be all that is needed to kill them. Any ideas?

Say that the characters don't start in the position alone but there is either another squad, some vehicle or whatever, and battle is announced when the tank burst through a bombed-out building and lands a shot on this squad/vehicle/whatever and the target is wiped off with one shot? Giving the players a hint what will happen if they get hit.

I think they could get sentinel with a pilot and a lascannon. It can die after one shot from ork kannon and it can deal significant damage if used with caution.

Characters won't be able to use it freely, though. Squad commander will give orders by using Command skill and it's test will reflect how well the order was performed. I was thinking that every DoF or DoS grants -10 or +10 to all tests made by sentinel.