The fellblade super heavy most definitely is in
IA2 2ed and that is the most recent rules.
I would avoid melta as it can (and should) have immunity to melta from a cheap upgrade.
I assume you are talking about laser destroyers, fellblade cannon, and a possible demolisher cannon. However non of the weapons has AP1. The most damaging profile to leman russ tanks is S9 AP2 3" blast armour bane, 2 quad lascannons (4 shots at 48" is much better than 2 ordnance shots at 36"), and a 24" S10 AP2 ordnance shot. None of these has ignore cover on them so the amount of damage it can do against an
AM player behind an
ADL is going to be limited. Even with just guardsmen for cover for your russ' the fellblade shooting everything at your tanks has a 25% chance to destroy a tank a turn. You can take 4 vanquishers for the price of a fell blade and deal out an average of ~3
HP a turn without any extra support (much more with prescience, putting your tanks on both board corners for side armour, and I factored in a 5+ cover save for a tank which will struggle to claim it). The fellblade's damage output is drastically reduced by cover.
Also your comment about the opponent's having other resources is a bit pointless. Of course they do...doesn't your side have more resources than 540 pts worth of leman russ'?
There are a number of options you have in the
AM codex all of which are great.
*Lascannon Sabre platforms with orders to give tank hunter are brutal, you can get 2 units of 3 sabre platforms, yarrick, and a skyshield landing pad for the same cost as a fellblade; averages ~4
HP of damage a turn and the fellblade will struggle to remove even a single sabre platform (they might chew through your ablative wounds though)
*bastion breacher shells on some form of medusa siege gun (I like the heavy artillery version)
*Guard Blobb w/ 5 lascannons,
ADL, Yarrick w/ Icarus lascannon, primaris or two. A bit more expensive (though you probably were bringing a lot of it anyways) but with the
ADL you will take relatively little damage (
GtG for a 2+ and order them up again if you fear you will get wiped). The damage is pretty great with ~4
HP a turn average (assumes prescience and tank hunter).
*rapier battery
*A bunch of deathstikes or manticores could work and would be pretty hilarious, just block
LoS
*A vlador tank hunter can be useful in a no D weapon environment. It is fairly easy to claim cover and you have a lot of psykers to help you hit.
*Warhound with overload plasma blast guns is actually pretty brutal, if will likely chip off ~3-4
HP and kill everything near the tank
If the opponent is always bringing it forward to use the demolisher cannon then there are even more options. Also if the opponent doesn't take ceramite then you can melta it to death in a variety of fun and interesting ways.