You can Special Summon this card (from your hand) by Tributing 1 "Airknight Parshath" you control. If this card attacks a Defense Position monster, inflict piercing Battle Damage to your opponent. When this card inflicts Battle Damage to your opponent: Draw 1 card. While "The Sanctuary in the Sky" is on the field and your Life Points are higher than your opponent's, this card gains ATK and DEF equal to the difference in Life Points.
By tributing Airknight Parshath from your field, you can special summon Neo-Parshath, the Sky Paladin from your hand. It's basically replacing a monster with a better one. You can also use Court of Justice to special summon Fairy-Type monsters from your hand if you control a Level 1 Fairy-Type monster like Freya, Spirit of Victory. This allows you to directly summon Neo-Parshath without having to use Airknight at all.
By having The Sanctuary in the Sky in your field, besides from having no battle damage taken from battles involving Fairy-Type monster, Neo-Parshath, the Sky Paladin gains attack equal to the difference of life points between you and your opponent if you have higher Life Points than your opponent. So the higher your Life Points, the higher the attack gain. Cestus of Dagla is a great card to be paired with since it increases the attack of a monster equipped with it by 500 and gives an effect to gain Life Points equal to the damage inflicted by it. Guardian Angel Joan is also a good match since it's a monster that lets you gain Life Points equal to the original attack of the monster that it destroyed.
Aegis of Gaia increases your LP by a considerable amount. If you have The Sanctuary in the Sky active, and you use Gaia when you and your opponent have the same amount of LP, then Neo-Parshath will get a 3,000 attack and defense increase.
Solemn Wishes or Spell Absorption can be an easy and steady 500 LP gain per turn. It can be a little slow, especially if you can't stall the duel long enough, but in combination with other ways to gain LP, it will add up. It's better to choose 1 or the other, instead of bringing both in a duel. Having multiple continuous spell/trap cards will only end up clogging your field. Use these with cards like Mausoleum of the Emperor to help summon Neo-Parshath easier/faster.
How to get
Fairy's Smile If you normal draw a Fairy-Type monster card during the Draw Phase, the card is shown to the opponent and you recover 1000 Life Points.
Fairy's Smile will help with gaining some LP if you are using a Fairy oriented deck, while Three-Star Demotion will help get Neo-Parshath out early. Deal damage and draw into your recovery cards for some sustain.