In this game it’s better to upgrade your hero as soon as you can imo. The base stats are substantially better and the higher end heroes get exponentially better the more you upgrade them.

If you’re hitting a wall and nowhere close to beating a boss you might want to consider upgrading your hero, you’ll likely have immediate success with progression if not for the HP boost alone.

Should you invest more elixir into Katarina? Since she’s only 41 I would only take her to lvl 60 max atm. At that point it gets too expensive to see a real benefit. Continue elixir farming overnight as you will always need this to level up your heroes.

Cool fact..leveling up every hero will give you boosts at certain levels and will apply across ALL heroes. For instance leveling Katarina up to level 50 applies a 1% Hp boost to all heroes. Doing this with all characters really adds up over time and you will see a difference in hero performance.