I only have 2 builds for the two most frequently used legendaries:
Chakram Spam: the usual spam chakram around tight corners to poke and annoy the hell out of your enemies. And the occasional spam Q to drive away anyone diving you.
Heavy Blade burst: Turns your heavy blade into Dredge's Abyssal Spike that can be exploded at will. This is for those who opt to use their Heavy Blade as a burst damage tool. Most people underestimate how much damage the blade itself can do while running this legendary, well time to give them a reminder. Throw the blade at an enemy, dealing 250 damage while hitting the ground or a nearby wall, then immediately trigger it for a nice extra 850 damage (no/barely any falloff, total of 1100 damage in the end). Note that the AOE explosion does not cause slowdown, but the initial throw still does if you hit people with it.
Majority of the card points are spent on self-sustain for your "I win 1v1" button and reducing your blade cooldown if you're running the heavy blade burst. Filler cards are up to you.
Vicious Assault in theory also gives you a decent amount of burst damage, but unlike Buck, your damn leap has a long windup and is a hell lot slower than Buck's leap (not to mention Buck has better sustain in general in terms of Bulk Up & resists on leap/heal). Works well against casual newbies, otherwise you'd most likely die, or in best case scenario, trade one kill.
Notes on some abilities:
Your chakram does not suffer damage falloff, but will deal reduced damage (500) on bouncing.
Triggering the blade explosion is a lot faster than the default recalling blade than you'd think.
You can throw chakrams and heavy blade, triggering blade recall/explosion while leaping in the air.
Combat Trance (Q) is Buck mini-ult on a 15 seconds (actually 11 seconds since the cooldown starts as soon as you activate it) cooldown. This is your "I win 1v1" button, better abuse it as much as you can. Try not activating it while you're low on ammo though... (or use Crowd-Pleaser card if you get into that situation too often, but please try not to)
His ultimate can charge very quickly. Using it for damaging enemies, however, isn't recommended unless they're already low and you need to chase and finish them off, as it doesn't apply Cauterize nor affected by Wrecker. Left click will swipe you in any directions (including vertical) while dealing 650 damage (same as your chakram) to anyone in the way, mostly used to chase down weak enemies or a quick mobility option. Right click will have you do a long ass jump spinning animation before touching the ground, dealing a massive 900 damage to those who are too slow to run away from it. You're invulnerable while spinning, but the moment you land, have fun getting burst down if enemies are still around and aiming for that moment. You're also not CC immune, so stuns and Jenos grip/Grover root can screw you over royally.
