Damage would be reduced to 1.
And I've tested it without masquerade, with poring hat instead of devi hat (Same reduct, but pink so cute).
Tested vs. poison with bathory (50% elemental resist poison), and 2x Myst card on cloak (60% resist poison).
Also tested vs GR with GR (75% elemental resist neutral) and 2x Devi on cloak (60% against neutral I believe).
The sinx's damage was reduced from 50k+ to like 300 per hit;
The champ's damage was reduced from around 4-5m to ~12k per asura.
Yeah..
Also slotted assassin robe adds 2% more reduct, so 95%.
Edit 2: One handed assaulter's spear (and decendant strategy, and some other battle set weps) adds 20% resist demihuman, 10% more than combat.
Slotted combat greaves add 1% more reduct, so 96% for non-pally/LK.
Commander's manteau adds 1% resist so 97%.
Edit 3: Masquerade is +3% damage to demihuman, not reduct.