Rhox Faithmender + Celestial mantle.
I'm not sure these combo, I think that Celestial Mantle simply changes your life total, not "gains life". For instance if you used Soul Conduit with Rhox Faithmender out, and increased your life total by strictly changing it out for a different one, he wouldnt cause you to gain double the difference. I think it works the same with the mantle. It just changes your life total, not gains you life per say.
Rhox Faithmender + Tainted Sigil or Rest for the Weary has potential to gain lots of life quickly though, for cheap.
You're wrong on both
Soul Conduit and
Celestial Mantle. Refer to their rulings on Gatherer, as well as
Resolute Archangel.
Soul Conduit: When the life totals are exchanged, each player gains or loses the amount of life necessary to equal the other player's previous life total. For example, if Player A has 5 life and Player B has 3 life before the exchange, Player A will lose 2 life and Player B will gain 2 life. Replacement effects may modify these gains and losses, and triggered abilities may trigger on them.
Celestial Mantle: If a player's life total is doubled, that player actually gains or loses the necessary amount of life. For example, if the life total of the enchanted creature's controller is 14 when
Celestial Mantle's triggered ability resolves, the ability causes that player to gain 14 life. Other cards that interact with life gain or life loss will interact with this effect accordingly.
Resolute Archangel: If your life total is less than your starting life total, you actually gain the appropriate amount of life. Other effects that interact with life gain will interact with
Resolute Archangel’s ability accordingly.