User blog comment:Xilinoc/Ch. 588 - The Headless Star 7/@comment-6657474-20140712060641/@comment-5398287-20140712185041

Here's the thing; if the Zanpakuto filler arc is to be trusted, then the Reiryoku absorption ability can be stopped if the vines are cut, thus breaking the connection, as seen to the right. As such, unless Yumichika is completely isolated from other combatants (as he was during his fights with Hisagi and Charlotte), he runs the risk of another Sternritter interrupting and cutting the vines, and that risk is greater than it would be against, say, a lieutenant like Hisagi because Giselle likely has captain-level Reiryoku, which is much greater than that of the lieutenant-level opponents Yumichika's faced prior.

There's also the whole thing about him not wanting to reveal the true nature and power of his Shikai to anyone (apparently including Ikkaku), so unless Ikkaku is killed or knocked out, he won't use it.

My point is, he COULD be the one to defeat Giselle without cutting her, but it would take some special circumstances for that to happen.