Forge world recently released 40K rules for the Mastodon Super-Heavy:
Imagine three 10 man Crusader squads (with PF/LC Sword Brother and extra PF on Initiate), plus Helbrecht, Grimaldus with two Cenobytes, and 6 Sword Brethren (i.e. Honor Guard) with Relic Blades and Banner of the Emperor Ascendant all loaded into the Mastodon… Turn 1, move up 12″, then Flat Out another 6″…. Turn 2, move up 6″, disembark everyone another 6″, then activate Helbrecht’s Crusade of Wrath and charge 2D6″ (with re-rolls because of Fleet) with all 40 models.
Everyone has Fearless/Hatred from Zealot and FNP from Grimaldus’ Cenobyte Servitors, plus everyone within 12″ of the Banner have +1A, so even your basic Initiate has 4 S4 AP- attacks (with re-rolls to Hit) on the charge (becomes 5 attacks if they suffer a casualty from Overwatch), while your squad leader Sword Brethren are rocking 5 x PF or 5 x LC attacks each and the Honor Guard Sword Brethren are hitting with 4 x S6 AP3 attacks each, again, all with re-rolls to Hit.
In terms of survivability, it has AV14/14/14, 10HPs!!!, Immunity to Melta extra dice, and 2 Void Shields (plus Super Heavy rules, which mean that any Explodes result just inflicts another D3 HPs, vice destroying it). Thus, the Mastodon is totally immune to S7 and below attacks (except for Lance, Haywire, and Gauss weaponry, of course), which takes A LOT of the potential enemy arsenal right out of the game.
So, that means that, AFTER stripping the two Void Shields, it would take an average of the following to destroy the Mastodon:
-45 x BS4 Lascannon or Grav Cannon/Amp shots
-90 x BS4 Melta or Krak Missile shots
-30 x BS4 Railgun or Bright Lance/Dark Lance/Blaster shots
-18 x BS4 Haywire Blaster shots
-90 x BS4 Gauss weapon shots
-90 x S8 PF/TH attacks in close combat
-30 x S10 Wulfen TH attacks in close combat
So, really the only things that a Mastodon are really vulnerable to are Assault Centurions (Armourbane S10 attacks really do a number on it), massed Haywire shooting (which is pretty hard to come by in 40K, outside of Kataphron Breachers and Skitarii Vanguard), and VERY massed Necron Gauss shooting… that and of course D shooting/attacks (i.e. Wraithknights, Wraithguard, Imperial Knights in close combat, etc.)… everything else is going to have a very difficult, if not impossible, time bringing down the Mastodon before it delivers it cargo of Zealous BT Crusaders into assault.