For free to play, it's 100% trade grinding: get some tradeable packs (mostly from tournaments) -> trade them for cards -> trade those for more packs. Repeat to infinity. It's tedious, but it's way more time-efficient than playing actual tournaments is (which is sad). This is because tournaments reward random packs (getting a Sun & Moon is not the same as getting a Team Up) and because the rate at which you can gain tournament tickets is pretty slow and gated by the ladder rewards RNG.
When I first started this game, I tracked the value of my collection. Approximately 75% of my wealth was earned from trading and 25% was actually playing tournaments, and this was when I was using tickets until I had 0. Kinda sad if you ask me. Now I won't even bother to play tournaments until they make the rewards 100% the latest expansion pack (which currently is Team Up).