Personally, I think this would be a Drain. When the attack hits, it may or may not be affected by the UFO's "slickness".
I'm thinking something like:
25d6 Drain Damage Shield, 0-END, Persistent, Standard Effect, Always On, Limited (5d6 per point Dex roll missed by), OAF - Immobile (the UFO).
Every 5d6 on Standard Effect gets 15 points, so at 25d6 you're covered up to a 5-pt miss, which basically nullify any damage at all with the UFO's on DEF