The reason for the peki is the beginning laning phase in order to spam your spear shot for last hit or Harass ... like you said panth lacks Mana ... so early game its a must but late game you can easily kill Blue Buff for Mana Regen .... and Personally Late game ... your mana is not an issue anymore since your damage output is done through ur HSS .... which comes to Asray comment .. HSS is the best one to max first because it deals the most damage ....and the reaosn why i have crits is simple .... most TANKS using ur Ability combo will stand a bit longer then the squishies so having that crit increase will help your team take them down ... especially since your HSS passive allows you to 100 crit so Infinity is a must to add that Extra bonus to almost garuntee a kill ... Which comes with Youmuu's ghostblade for its passive to attack faster and Arm pen .... try the build, getting stacks is fairly easy and my friend used it on his first Panth game and went 26 -2 -13 ... i always go positive when you play him correctly

This build also heavily relies on your SOTO which, if you aren't for some reason getting kills/ living in order to maintain the stacks is completely useless.
Also... what are you doing with your meki pendant? you don't seem to build it into anything but as pantheon you really do need some mana regen for the entire game I'd say.

I don't like this guide. You max the HSS first, but your gear is build on crit. Unless you don't autohit, you won't be able to use it and because you skill yoour HSS you won't autohit so much. Don't forget: You can not autohit while using HSS unless you don't want to cancel it 'til you don't do anything.

It would be more effecient if you skill the spear first imo. You can harass the enemy and finish him off after that with your combo: spear > stun > autohit > spear
Most enemies won't stand after this if you harassed correctly.