I used one of the exact decks that jappert listed in his link, the monolife rush one, except I used 6 pillars/6 pendulums to defend against earthquake, when I was new and grinding ai3. It worked well enough that I'd upgraded a lot of it in a few days of heavy playing. The biggest weaknesses were shields like ice shield or dimensional shield, though sometimes if you got a lot of adrenalines and a bit of luck you could still punch through ice shields and win. Dunno if it was the best, but for an unupgraded deck, it worked pretty darned well. One of those immolation/phoenix things that people suggested would also work very well in ai3.
As for Miracle, you can get it by winning a rare spin in the arena sometimes; that's probably the most reliable way. A few false gods have it (Miracle, Rainbow, Paradox) but if you want an ai3 grinder, you likely aren't ready for false gods yet. AI4 sometimes has a miracle, but getting one there would be low odds. You could maybe win one in the lower ais (1-3) but I don't know if they have random decks (I know ai4 is random) or if they are predesigned decks, or if they even have rares. You could also win one in a regular spin in the arena or in pvp if people have it in their decks, but again that is unreliable; most decks that use miracle probably don't use very many due to the high cost, and the cost makes it hard to use in most decks.
