Little question about the buster only rules: must i clear the game without using a special weapon or is the rule only for boss fights?

This is the general speedrunning forum, no one here knows anything about Mega Man. You need to ask in the forum for that specific game.

Megaman has their own site for leaderboards and resources. Would probably recommend joining their discord under the resources tab on their site https://megamanleaderboards.net/index.php

My first try to beat this game as buster only speedrun have fun watching your Kiki0231. https://megamanleaderboards.net/index.php2048