An AK-306 won't have the penetration power and by the time you try to call in a missile strike lots of damage can be done and you assume it would be missiles.
It is a landing ship... an AK-306 would only be there as a sort of HMG type set up on a US ship.
The actual gun appears to be a duet which has two 5,000 rpm 30mm six barrel cannon with the same ammo so the same penetration... but much more effective against small fast targets.
Helios are also easy pickings for MANPADS as shown in Syria when the Jhads downed Russian birds easily with them. Also what if the enemy can jam your missiles?.
Tactics and the right equipment can make them effective... besides the current missiles that will be carried by Russian helicopters like the Izd 305 have a range of 25km... which MANPADS can cope with that?
Drones will be used to monitor the landing and any resistance to the landing will be deal with using naval gun fire support no doubt... helicopters could be used to support the attack which could include dropping forces behind the beach defences to attack them from the rear and destroy their support lines and rear areas.
You cannot Jam a Gun, so IMO a support gun is better than AD on a ship whose job is to deliver troops to a beach. head
The only calibre gun that would be useful would be a 152mm gun which would take up an enormous amount of internal space on such a small ship.
A 40K helicopter landing ship might have a 152mm gun mount to support operations but these little ships will just have air defence guns and missiles I suspect.
If there is a dug-in enemy that survived all the aircraft, drones, missiles and all the artillery fire from the rest of the fleet then a single gun on a landing ship is pointless because you already screwed up the landing long before that landing ship approached the beach
If a landing is screwed up because there are living defenders opposing the landing then there have not been many successful landings that weren't screwed up.
I seem to recall a US landing on the Alutien islands in about 1942 or 43, where they only lost about 30 men, but there were no Japanese on the islands they landed on so it was a totally unopposed landing which by your criteria makes it not a screw up.
Thing is that you don't need landing ships if the landing is going to be unopposed... just transport ships and floating piers...
Some dug in troops can't be defeated with air or naval gun power... but it keeps their heads down while you fight your way onto the beach and spread inland and take those positions with ground forces...