Mt. Fisty is an absurdly difficult to climb mountain in the southeast portion of the game world. You can get there by going southeast out of Freedom City into the juicer camp, crossing the bridge, and continuing southeast until you pass the crater rim helipad, at which point you take the NE path. You will eventually arrive at a valley. Go east past this valley, and there is the base of the mountain.
The Guru is located in a cave a few climbs up from the base of the mountain.
The mountain consists of several 'nearly impossible'-grade climbs, two of them being 50 feet long. All other climbs are in the very difficult - nearly impossible range, and are of varying lengths. The final gauntlet at the top - the Coronet - is exceedingly hard.
To successfully climb Mt. Fisty you need at least:
At least 20 climb skill total. Cold-weather clothing. It can reach -45 degrees Fahrenheit at night near the top. Shitloads of grappling hooks, or a buddy with leapfrog to toss climbing ropes down to you. At least 20.
It is also recommended you have a few raws in throw, so your grappling hooks actually work.
You'll probably fall and die or run out of grappling hooks, or do something stupid and fail the first time you go.