I can not imagine any lifeform (other than very, very, very exotic), that survives a burst from an MG.
The „armor“ of the creature has to be of 1 cm solid metal.
Do we need another concept or more fantasy?
I don't want to disappoint you but 1cm of metal will not stop MG military-grade ammo. Especially from vehicle-based MG :)
Any water-based large lifeform will have zero chances against modern military - even rifles will deal mortal wounds to it (as bullet hits lead to necrosis) plus artillery & anti-tank weapons will easily kill them. People are tricked by endless action movies as in real life most of times a single bullet hit is a ticket to hospital.
And game already is a heavy fantasy. Solar plants available before laser or at least electric engine techs and ignoring planet orbit radius, "slavers" and "raiders" instead of farmers on lifeless planets, howitzers as separate option for tank design, 300mm guns for not mechanized artillery, tactical nukes researched before strategic ones, current values of ammo consumption... All this is a pure fantasy.
My IMHO - combat treat from alien lifeforms should be an option, like cults and so on (or at least "powerful lifeform" as an option and without it animal stats should at worst be equal to militia infantry). It add nothing to game, it's completely unrealistic (same as cults with stat boosts and major corporations in post-apocalypse world). Someone will like it, true, but for me it's a big facepalm when I see pack of animals having soft attack value of artillery or MG.
Right, the 1 cm of metall was only a picture of the problem.
A little bit more „realism“ regarding firepower, damage or sequence of develpoment would be welcome.
The imagination of the struggle for survive on a dangerous surface of a planet should not be damaged by
totally unrealictic pictures. And organic lifeforms attacking MG or flamethrowers is a little bit to much.