Edit: This solution has been vetted and will not be taken down. Reporting it wastes both your time and the site mods' time. See the comments below around comment #65-70 (depending on if any earlier comments get deleted) for a direct response from a mod requesting that people stop reporting this.
Ctrl+F search for Palesius to jump to all his helpful comments below.
This works for all titles that have been added to Halo PC.
343i officially supports mods on TMCC for PC. Mods do disable achievements. We have a news article right here on TA discussing this. However, via oversight or error they left the cloud save shared between versions. You can take advantage of their slip-up to pop achievements on the non-mod version on PC after playing on the mods-enabled version. (afaik the save doesnt transfer over to Xbox anymore)
The short version:
1. Play the LASO playlist (or any individual mission if going for solo legendary, par time, par score, speedrun, lone wolf 150k) with mods enabled.
2. Save and quit at the epilogue for the LASO playlist, or at the end-of-mission cut scene for any mission except Pillar of Autumn which must be played from the final checkpoint at the MAC cannon. For Lone Wolf there is a checkpoint at the cut scene after you die.
3. Load up your saved mission or resume the LASO playlist on the non-mod PC version. (One commentor stated that individual missions must be resumed on the same platform so if you save/quit an individual mission on modded PC, you must complete it on non-modded PC. I cannot confirm this personally.)
4. Skip epilogue and die on lone wolf for LASO playlist. Finish the mission if doing a campaign mission (par time or par score). Note: For the solo legendary and legendary speedrun achs this means you'll need to switch back and forth and do one mission at a time.
I can't link you to what mods to use.
1) I dont even know what they are and
2) I want this solution to stay up.
For more detailed and up to date info check the forums for a helpful thread on this.