Remilia's too concentrated on screaming "I'M THE FUCKING VAMPIRE I'M STRONGEST AWESOMEST etc. etc." to keep an eye on Suika. Plus, an oni's a lot stronger than a vampire (physically), so Suika can just dissolve the Gungnir with her ability and give a big punch in the face to Remilia. Not to mention Remilia's clothes look much more damaged than Suika's, so probably she got beaten up and is just trying to get some glory by fighting back.
Being physically stronger doesn't mean much if the opponent is so stupidly fast that she went around the (fantastic) Moon in a matter of seconds and can turn into mist/bats to dodge attacks, so simply trying to punch her would fail anyway. If Suika used her powers then it would work, but that's just because her ability is broken as fuck.