Sigilyph GX is interesting. In a Basic GX heavy format where almost every single basic GX that is or will be meta relevant can hit enough damage to OHKO themselves, it makes for some interesting trades. Also Stage 1s swinging for 2 hit KOs will hit themselves for 110-150 which allows Sigilyph GX to take a tactical OHKO the following turn.
Some fairly simple solutions to this card is to just play higher HP Pokemon like Gardevoir GX and simply OHKO them with the minimum possible amount of energy. Solgaleo GX's Sunsteel Strike also allows it to trade positively with it. At the same time, even doing this will result in
And of course the simplest solution is to just use non-GX attackers. There are more and more non-GX attackers who can hit for 190 damage like Xerneas Prism and Gardevoir right here in this set.
I would recommend Sky Pillar for Sigilyph GX decks to protect the ones on the bench from Pokemon like Tapu Koko Promo, UNGX's GX attack, Necrozma GX's GX attack, etc etc.
Will be interesting to see where this deck stands when it comes out. It's not like people don't run non-GX attackers, and outside of the GX attack, Sigilyph GX actually has a very hard time OHKOing most one prize attackers.