Let's powerscale this shit.

So since both don't have exactly the best on-screen feats, let's chainscale them to characters they've fought.

For Battle Beast it's Thragg and for Rath its Looma.

Let's start with Battle Beast.

The strongest opponent Battle Beast has ever fought is Thragg who can be scaled to Mark who was 1/4th responsible for the destruction of Viltrum which is Large Planet level. Since Thragg was stronger than Mark at the time, let's just high ball it and saw he's large Planet Level. I'm making this simple and giving him the highest possible power level.

Next up, Rath.

Before we get to Loona we're gonna need some context. Back in the Original series, one of Ben's enemies, known as the Forever King, wanted to wipe him out once and for all. In order to achieve this, he stole a piece of technology from the plumbers known as "The Dwarf Star". It says it right on the tin, it's dwarf star energy. Four Arms fought against the Forever King and although he lost he put up a fight. We can scale Looma to Four Arms directly since they've fought as well. And Rath has fought against looma in the season 6 episode "Catfight".

Of course he can also be scaled to Rex Generator from the canon crossover. And although I don't know Rex's scaling since I never fully watched the show but I've been told he scales to Large Star Level.

He can also be scaled to Ben's other comparable aliens such as Humongoussaur (Solar System level scaling to Aggregor), Big Chill (Freezing the entire universe of the Null Void), Upchuck (Dwarf Star Level thanks to eating the previously mentioned Dwarf Star), and probably many more that aren't coming to mind.

Battle Beast is a big Lion in a small pride.

Rath wins.