This was an exercise in seeing just how much life I could gain at once, and what the resulting creature would look like. It's rather ridiculous. With the combo listed below, you can go from 2 to over 5000 life in a single combat round, and do almost 250 damage to your opponent in the process. It's stupid. I don't think I'll ever see the combo happen, given the number and CMC of the relevant cards.
This is all about holding out long enough to get your stuff in play. Here is the combo you really want to have out:
Serra Avatar enchanted with Celestial Mantle. (And Optionally equipped with Loxodon Warhammer for the trample)
2 of either Boon Reflection or Rhox Faithmender (In whatever combination, this makes all life gains quadruple their value)
True Conviction
Cradle of Vitality
8 untapped Plains.
This requires your Avatar to get through unblocked, or to trample some damage over. I'm considering swapping in some pro-creature instants specifically for that.
So if you can use this combo, here is what happens, assuming you're at 2 life starting. WARNING!!! MATH AHEAD!
*You attack with your 5/5 Serra Avatar and make it through to hit your opponent. Because the Avatar has Double Strike, they do 5 damage first, gaining you 5 life. Due to your 2 life doublers, you actually gain 20 life.
*This brings your life total to 22. Immediately tap 2 mana for the Cradle of Vitality to add 20+1/+1 counters on the Avatar, making it now a 45/45 (22 life, +3 from the celestial mantle, +20 counters) creature.
*Because you did damage to the player, Celestial Mantle triggers, doubling your life total, bringing your life to 44. But wait! This also counts as life gain, and is quadrupled because of your doublers, so you actually added 88 life, not 22, making your life total 110. So tap 2 more mana on Cradle of Vitality and add 110 +1/+1 counters to Serra Avatar, making it a 243/243 (110 life, +3 mantle, +130 counters) creature.
*Normal damage segment triggers. You do 243 damage to your opponent, gaining 972 (243*4) life, bringing you to 1082 total. Tap 2 mana to add that life gain to your avatar in counters again, bringing your Avatar to 2187/2187 (1082 life total, +3 from mantle, 1102 +1/+1 counters).
*Mantle triggers again, adding 4328 (4*1082) life to your current total, bringing you to 5410 life. Tap your final 2 mana to add that in counters, bringing your avatar to 10,843/10,843 (5410 life total, +3 from the mantle, and 5430 +1/+1 counters).
So in total, you gained 5408 life, and did 248 damage to your opponent. Congrats. Add a zero to all of this if you started at 20 life.
Oh, and as a bonus, your pridemate(s) gained 8 counters. Whooptee do.