I assume Frosty is manifested, otherwise the missiles are useless unless X-Y is also ethereal.
X-Y does not seem to have spell resistance, because it does not resist the two missiles. They inflict 7 points of damage. X-Y has 16 HP remaining.
Round 289: X-Y dives down to 60N20E20Z. X-Y has seen much stranger whence it is from, so it is not horrified by Frosty's appearance (rolled 17 Fort save).