A face-down "Big Shield Gardna" can only negate a Spell Card that targets 1 face-down monster, so it cannot negate "Riryoku", "Creature Swap" or "Dark Hole", but could negate "Nobleman of Crossout" or "Tribute to the Doomed".
If "Big Shield Gardna" is targeted with a Spell Card, you may chain "Book of Moon" to flip it face-down, then it can negate the Spell Card and will flip back to face-up Defense Position.
"Skill Drain" only negates the effects of monsters that are face-up on the field, so a face-down "Big Shield Gardna" may still negate a Spell Card that targets it while "Skill Drain" is active on the field. It will still be flipped to face-up Defense Position, and now its effect that changes its battle position when it is attacked will be negated.
If "Prickle Fairy" attacks a face-down "Big Shield Gardna" both their effects will occur at the end of the Damage Step. "Prickle Fairy" will be Step 1 and "Big Shield Gardna" Step 2. So it will change to Attack Position, and then the effect of "Prickle Fairy" will change it back to Defense Position.