Thanks, I've been lurking for a while here, just never felt the urge to post until now
Yes, the player will always be smarter, which is why arena decks are allowed the FG-like advantages as a handicap, and which is why you'll have to build your decks for the AI. Generally, don't expect the AI to hold any cards for combos, and knowing their targeting priorities is always good. Throw your deck in the trainer and play against it. Does the AI do what you expect it to do? I've had a deck idea tank because of that; the AI prioritized fractalling arctic octupi before pufferfish, screwing up my intended win condition
Your deck will only lose lots and drop out IF you let people beat you. Gold league still has some very meh decks along with the mean decks. The whole point of the ranking system is to weed out the weak and keep the strong. Decks dropping in rank has to do with a combination of people submitting new decks, and the deck itself losing too much compared to the average. When decks are tied with the same rating, they are arranged by age. Older decks will be ranked higher than younger decks with the same rating. If you want to keep your deck in the top 500, the answer is to build a strong enough deck that defeats most of its challengers. Because of the way ratings work, the more new decks submitted each day, the higher your W-L ratio needs to be to stay above the inrush of newcomers. Also, the only way to get into the top ranks of a league is to
not submit a new deck each day, as no matter how good a deck's W-L ratio is, it'll never build up enough rating in one day to reach the top.
Test play a few popular decks against your deck, use the trainer if you have to (I know the level 6 AI is stronger than gold league, and it auto-ups your cards for you, but usually it's close enough to find out if the AI behaves well or not. You can tweak the quanta balance and such afterward). If your deck consistently gets beaten, then find out why and fix it. Remember, you don't have to account for all counter decks, if your deck can rip through the majority of deck styles you throw at it (rush, control, stall, hope, creatureless, etc.), then it should be robust enough to survive in gold until people come up with new farm decks. That's the fun of arena though, think of it as an arms race between the farmers and the deck builders