Counter Tips

Bowser's only landing tools are his downair and bowser bomb, both which are easily readable and punishable by either uptilts and baiting them out, or wait till they land and hard punish with an fsmash

Bowser's upb is a great out of shield, combo breaker tool with decent range. Fortunately, it doesn't do much shield damage and it's easily punishable if shielded.

Don't die. If you don't die then you have a much higher chance of winning.

I personally don't like fighting Bowser as any character, but all I can say is to primarily focus on charging your limit. Then keep using side B. Nair also works wonders.