You need to be patient, which can be difficult for sure, but if you need help strategically I would recommend asking:
Are you maxing out your healing cards before leaving the first battle?
Are you minimizing your roll cost to 1 before leaving the first battle?
Are you making sure you have back-up roll cards/ the enemy is doing minimal damage before rolling a new max hp?
Are you making sure to stall out fights so you have time to level up, rather than winning them?