What's the polite way of saying "I'd sooner chew glass."
Seren is so far removed from having remotely the right mindset to approach this type of thing that Owl sees no easy way to correct her.
Owl could say "I don't know what you mean by 'about the same level as lighting your sword on fire' because a) that is ridiculously underspecified (are you burning the sword? or are you creating fire in its vicinity? if the latter, how hot is the fire in its vicinity?) and b) I'm guessing that your other powers which are 'on that level' aren't eg outputting the same number of watts."
She could ask Seren to precisely detail the two or three weaker powers that she wants to test, in as much detail as Seren knows, and then decide whether Owl knows enough to safely help test them.
But... if Seren is the type of person who would would propose this in the first place, and not immediately see what she's doing wrong, she's going to do something stupid at some point, in some way that Owl hasn't anticipated, and Owl very much does not want to be caught in the blast radius.