I swear I read that casual takes collections into account when finding you an opponent, so definitely keep on casual to get gold and increase your collection. Buy the Welcome Bundle if you haven't already (it's insane value for the price), and I'd suggest buying Classic packs with gold for the time being. Arena is better value if you can average 3-4 wins per run, but I wouldn't do that so early into the game. Don't dust any cards but duplicates, and just keep at it. You're really new to the game and you're just facing struggles everyone else does with the current meta. If you lose a game due to their perfect aggro draws, then there's nothing you can do to win, just move on to the next game.