I recommend using Rabbid Peach and Luigi, which I believe is the strongest team combo in the game. I re-speced all my team members to power up abilities and as few points as possible into slide/jump attacks. Mario’s fully upgraded M-Power comboed with Luigi’s fully upgraded Steely Stare is an extremely effective way to deal out a ton of damage. The stats shown at the start are actually my stats AFTER completing the final boss, so it may appear as if my characters a bit more powerful than they actually were.
Also, if you can afford them, purchase weapons that have bonus push or bounce chance against valkyries.
Bowser has his own overwatch ability, indicated by the glowing fire under his feat. Use the toy car to draw overwatch to the car instead of a teammate.
Focus on killing the valkyries first before attacking bowser.
Bowser’s 1st turn will be a focused attack at a single team member, so be sure to have everbody behind full cover
Bowser’s 2nd turn will be a “siren song” followed by a AOE ground stomp. Spread your team around the arena as far as possible on your second turn to avoid getting everybody smashed.
Bowser is easy to avoid, so focus on the shield carrying bucklers. Bowser will cast his fireballs and probably burn your dudes and ruin your cover. Move your dudes to the middle in anticipation of the spawning enemies
Do not worry too much about cover and focus down the shield bros before attacking bowser. Every unbreakable (for the moment) cover gets hit by fireballs and the splash damage will hurt and push you.
Attack bowser to damage the shield caster without sacrificing position
Bowser is invincible when surrounded by his glowing red orb
Keep the concrete barriers between you and Bowser. Bowser will spin on his shell and charge a random teammate, and when he hits cover, will stun himself.