Well, talking of the Xenomorph's lore If anyone is even interested or care anymore... Their chitin armor is made of polarized silicon, providing a hard shell and being space proof. However even it maybe resistant to heat, it is also vulnerable against intense temperature change; If it is heated up in a sudden, the chitin armor will stretch out and giving a massive pain to the Xenomorph and even giving shock, so goes for intense temperature drop but the armor will shrink instead this time. What does it means?
Cook'em up with the flamethrowers.
Always worked in every Alien series.