First, ensure that any data entered by the user is being temporarily saved. It's horrible to go to effort of filling out a long form only to have everything erased because a session timed out or you accidentally left the page.
Consider using inline validation and perhaps encouraging statements. They shouldn't be patronizing or too chummy. But showing a percentage progress and something sympathetic like, "We realize this form is long and appreciate your patience in completing it. You're over half way done." might ease the pain.
As much as you can, still attempt to visually separate and group related elements within the form. There's always a way - whether it's a few more pixels of space or a subtle variation in color in alternating sections. Be especially attentive to alignment and field length. All forms should be well designed. This goes double for monolithic versions.
You mentioned mobile and that means small screens. If you are using validation it's good to mention near the bottom specific issues. Otherwise the user has to hunt through the form or may be frustrated when there's no indication why the form won't submit. This just gets worse when you can't see the entire form at once.