this is a late comment as I'm currently working on doing hunting horn solo runs (with gear only available before any given monster that's being hunted). one thing i realized later on but didn't comment on in this recording is that i used Velkhana Y-armor in this attempt (only available by slaying Arch-Tempered Velkhana, arguably harder than Alatreon or even Fatalis). though i did end up putting out a recording showing that it's possible to clear ATvelk with a pre-Fatalis, pre-Alatreon loadout (ofc pre ATvelk) and solo, you can absolutely and just as comfortably clear Fire-Active Alatreon with the standard Velkhana-B armor set available without slaying Arch-Tempered Velkhana, and only sacrifice a few levels in Evade Extender/Window at best. (the back-up set shown in this vid with my Kjarr-fire Swaxe is only really there if you fail to slay Alatreon once all horns are broken and get carted by a following Nova--with you wanting to switch sets mid-way for whatever reason, otherwise, you can just keep smacking his head which is a good hitzone for raw damage as he'd already be fairly low by then, siege-weapons are not necessary but do help) on the topic of needing to suffer through a 2-week period in between Safi'jiiva and Kulve Taroth rotations (and horrible rng), if one were to progress through Alatreon, Fatalis, then Arch Tempered-Velkhana in that order WITHOUT siege weapons, you can hunt Fire-Active Alatreon with Frostfang weapons, then hunt Fatalis with Alatreon weapons, then finally Arch Tempered-Velkhana with Fatalis gear. however, if you want to attempt Arch Tempered Velkhana with pre-Alatreon, and pre-Fatalis equipment, my best bet would either be using Rathalos/Glavenus-tree Swaxes for fire damage, or the Lightbreak Swaxe (Raging Brachydios), though i've not personally tried this myself. ultimately, progress through the game how you want, no shame in SOS-ing and getting help, these ramblings only really matter if you're as particular as i am about personal progression and challenge in video games lol.