Steffan, in the landing page editor, edit your Page Elements (like your form) with either
posOverride,
posOverrideRight, or
posOverrideFull to float the element for positioning. See below:
FYI:
posOverride floats left,
posOverrideRight floats right, and
posOverrideFull floats to the left with 100% width.
*Props to Edward Unthank from Yesler for identifying this technique. I'd recommend
contacting him if you need help with responsive Marketo LPs.
PS. BTW, depending on what responsive CSS you're using (e.g. Twitter Bootstrap), you can also adjust where your Marketo elements will be placed by editing your template. In the proper div Class (or article/section), you'll want to include
<?php echo $mContext['bodyElements']; ?> for your landing page elements.