Best way grind.
L3 is better than PvP, T50 and L5 for speed grinding. Mixing it up is a good thing, but in general it is best to stick with L3.
Use a simple 30 card deck that uses only 1 pillar type (Mark can be different) and loaded offensive creatures that are cost effective (Graboid, Lycanthrope, Lava Golem, Horned Toad....) Think most players will end up playing a few hundred games before making the jump to final jump to False Gods, so it would be better to use several different decks to make the grind fun.
I tried a bunch of different decks, but had the best luck with this simple life deck. Out of 100 games, I got only 4 losses. Averaged 25 coin a game (not including selling cards) and most games lasted 8 or less player turns.
1 Short Sword
12 Emerald Pillars
6 Horned Toad
6 Cockatrice
2 Heal
3 Adrenaline
Other decks that worked pretty well.
Poison - Death + Water(Mark)
Lycanthrope and Abomination - Entropy + Darkness(Mark)
Graboid and Antlion - Earth + Time(Mark)
Lava Golem and Fire Spirit - Fire + Earth(Mark)
Blessed Pegasus and Firefly - Wind + Light(Mark)
Blessed Vampires - Darkness + Light(Mark)