For a mono white lifegain deck, consider looking into the soul sisters archetype for some ideas. At it's core it relies on dropping multiples of
soul warden and
soul's attendant, followed by large amounts of creatures to build up high amounts of life and beat opponents to death with
serra ascendant, a potential 6/6 flyer with
lifelink for 1 mana if you have 30+ life. An optimal combo would be T1;
martyr of sands. T2; activate martyr, reveal at least 4 white cards and have greater than 30 life and drop the 6/6
serra ascendant.
Other good cards to consider would be
peach garden oath, more
divinity of pride means more consistent aerial threats,
nip gwyllion can be stacked with
edge of the divinity for a +3/+3 buff for 1 mana, potentially landing a 7/7 lifelinker on turn 2 etc. Also
Cranial archive is strictly worse than
elixir of immortality, since the elixir costs 1 to cast, provides 5 life when used and goes into your deck with the rest of your graveyard.
If you want to go all the way with absurd lifegain, maybe add a couple of white and green dual lands for
ajani, mentor of heroes. Not necessarily effective but the +100 life may as well be a win condition because few people want to chip that away. A green splash can also net you access to
heroes reunion which at 7 life for 2CMC is about as efficient as you can get, as well as
essence warden if you try out the soul sisters style of drop loads of creatures for extra life. You could also try some to add
fangren marauder artifact synergy with cards like
sunbeam spellbomb,
scroll of avacyn and
bottle gnomes if you were to try out a green/white lifegain build.