Goku at any level below SS4 would be soundly defeated by Will Vandom if she were in Zenith form. Below is an explanation of just why she could defeat Goku in almost any of his power levels, if she were in her Zenith form. (note this is a tl;dr post if you want to skip this explanation)
Will has several forms (below listed in order of increasing power with added abilities and descriptions in parentheses):
Normal (ordinary human)
Normal 2 (ability to communicate with electronic devices)
Guardian (can fly, but has no offensive special attack of her own, and thus must
depend on team mates in battle, however she can seal "portals" with her crystal)
Guardian 2 (can shoot lightning out of hands)
Zenith (is made of pure energy, and technically no longer human, and her own soul can't even exist in this new body and so her soul goes off to an alternate dimension while she's in this mode, and all her attacks are at a super maximum overdrive level that is required to kill the "final boss" of the show, but she tends to get stuck in this form like a "berzerker" mode, and her soul in the alternate dimension must focus all her mind just to snap herself back out of this mode in order to return to being human, so she can go back to living a normal life and also so her soul can return to her body, as her Zenith form simply can't contain a soul).
We see Normal and Guardian forms in the season-1 episode-1. Later in season-1, Will in Guardian form can also open "portals" with her crystal after absorbing a special magical item into the crystal (originally she could only close portals). In the final episode of season-1, we see her able to shoot a ball of energy along (also her friends do the same), to seal away the "final boss" from season-1. Starting in season-2 episode-2, she can open and close "space folds" while in Guardian form.
We first see her Normal 2 form in season-2 episode-3. Here she all of a sudden starts hearing her calculator talking. In season-2 episode-4 she gains the ability to shoot lightning from her hands when in Guardian form, thus she has now reached her Guardian 2 form. Later in season-2, she discovers that her power is actually that of "quintessence" a powerful magical energy that can do more than just generate lightning bolts. It has the power to bring things to life.
In the final episode of season-2 we see her one and only use of her Zenith form, when fighting the "final boss" of when in the last episode of season-2, in conjunction with her friends also go into Zenith form (albeit, Will's Zenith form is quite obviously the strongest).
Below are screencaps of Will in each form.
Normal (Normal 2 is visually identical so no separate image)
Guardian 2 (showing her shooting lightning, otherwise visually identical to Guardian)
As I said before, Will in Zenith form would SO KICK GOKU'S A$$ (as long as Goku is in any form lower than SSJ4). Goku can SHOOT OUT energy, but Will who has the power to BECOME energy obviously wields FAR MORE POWER, and has an advantage over ANYBODY who is just composed of ordinary matter without the possibility of turning into energy.
If Goku were at SSJ4 though, he would probably equal to Will's Zenith form (possibly stronger than her form). If he "fusioned" with Vegeta to become Gogeta though, I must admit Will would need to get lucky to win even if she were in Zenith form. However I wouldn't entirely rule out the possibility that Will in Zenith form could still win even in such dire situation. Will is not only strong, but also smart and always seems to have some trick up her sleeve, and she's gotten out of situations with seemingly insurmountable odds before.