For THF, I'd actually recommend either /DNC or /RDM since you're on your own. Other than Arrapago, I'd suggest either Bhaflau or Zhayolm, with the following strategies:
BHAFLAU: Definitely sub RDM and bring a sword, ideally Vampirism, and if you also have a Malevolence, bring that too. /RUN works as well, but Haste is too good to pass up for a solo situation.
Floor 1: First thing is to kill the Flan or Wamoura, depending on which side. Wamoura you can steal Erratic Flutter from, but I find to be a bit more dangerous overall. Flan does have spells, but you can use Hide on it, Blink on ancient magic, and on some spells you can even get behind it and SA. It's smooth sailing once you beat either of these since you'll unlock most of your stuff. Get your accessories from the Fly or Wamouracampa and then just go up, ignore the goblins and dahak.
Floor 2: Skip to the next floor.
Floor 3: Farm if you want, but you'll likely want stats (HP is a must). With a good magic dagger (like Malevolence) and magic offhand (another, or Vampirism, etc), you can dish out some good Aeolian Edges, though I've never bothered to cleave that many gears on THF. Go up when you've had your fill.
Floor 4: Some people go for the NMs/Acroliths, that's up to you. Kill the 3 gears and pop Cerberus. When he's there, switch to your sword mainhand and use Sanguine Blade. Having a good Sanguine set helps not only your damage tremendously, but also survival. Barfire.
Floor 5: Same as Cerberus, really.
ZHAYOLM: I still like RDM as a sub for this because of Phalanx, Haste, Sneak/Invis, etc.
Floor 1: Be careful when pulling Toads and Crawlers due to how easily they link. Having defensive accessories like Defending/Dark Ring helps when stacked with spells like Protect, Stoneskin, and Phalanx. Kill a Mamool along the way if you need to unlock HP. If you can afford to, save some TP.
Floor 2: Just go up.
Floor 3: Just go up.
Floor 4: Just go up.
Floor 5: Your goal here is to kill a single gear, probably while having other stuff on you, with very limited gear. Pull a gear as best you can and pop Perfect Dodge. When you have TP, use Bully and SA your best WS (probably Rudra's). You have the duration of PD to kill that gear, though a Bully SA WS should be enough. Once it's dead, you'll unlock your important gear (if you're not using GearSwap, make a macro to re-equip everything). It should be smooth sailing from here on.
Floor 6: Kill the Gears and Ramparts (except the one that summons Hydra) in any way you please. Hydra isn't too challenging, he's arguably easier than Khimaira since Fulmination sucks. Baraero.
Floor 7: Same thing. Baraero.