Hello Mr Leige,
I doubt it would ever be a situation where Russ and all 3 Leviathans where in a position to counter a Deathstar (Primarch + Term bodyguard and support HQ of choice in a transport). You are talking about 65-70% of the Dudes army, commited to one melee. He might, if that situation somehow occured, defeat the Deathstar, but I would bet a shiney penny that he's lost he game. The rest of his army is a couple of characters and 2x 10 power armour Dudes... .
My issue is Russ. Either he is alone (and so the Deathstar drowns him in AP2 hits) or he is in a unit (reducing the units effectivness and still the 10x Grey Slayers, kitted as they are, are no match for kitted out terminators).
A Fire raptor would cost less than the 2x 10 Grey Slayers and would chew them up.
The walking Leviathans can soak up firepower, but some dedicated Anti tank (Laser Vinidactor spring to mind as a commonly seen/utilised option) will soon account for some if not all.
These aren't 'hard counters' but common things in everyday play :-)
And just as a measure, I threw this together using esimates of costs. Rouoghly done but could be switched for most Leginos. Nothing over the top, combo tastic or spammed to death.
LOW 450 Primarch (ROW Pride or Primarchs Chosen)
HQ 50 Centurion
TRP 280 5 Man Terms with some gear
350 + Dedicated Transport - Spartan
TRP 210 10 Man Vet Squad (Sniper for Outflank)
35 + Dedicated transport - Rhino
Elite 160 2x Rapier Batteries
Elite 160 Cortus Contemptor (Chainfist)
Hvy 220 Fire Raptor (Autocannons)
Hvy 150 Laser Vindicator
give or take depending on loadouts/legion around the 2000pt mark