Aside from Saitama, which other heroes defeated dragon-level monsters in One Punch Man?
There are three such unassisted defeats by two heroes in the manga.
Child Emperor defeats Phoenix Man in manga Chapter 99.
If you count fights which involved multiple heroes, but no Saitama, then Melzargard, one of Boros's underlings, was defeated with the combined efforts of Bang, Metal Bat, Puri Puri Prisoner, and Atomic Samurai.
And honorable mentions from the webcomic:
Gums and Fuhrer Ugly are both one-shot by Bang in chapter 72 (of the webcomic, of course, not the manga). This occurs during a large multi-person battle once the Monster Association and Garou fights move to the surface. Other heroes were involved in this fight, but given the devastating one-shot nature of the takedowns, you might still count them as unassisted kills. Gums had previously defeated both Pig God and (basically) Genos in one-on-one combat, and Fuhrer Ugly had defeated Sweet Mask. Also in the webcomic and during essentially the same sequence, Tatsumaki effortlessly crushes Gyoro-Gyoro. When she then reveals Gyoro-Gyoro's hidden true identity, Tatsumaki is essentially defeated by a cleverly arranged sneak attack by this hidden identity and other monsters. So it's up to you to decide if this was a solo victory by Tatsumaki against a Dragon class threat or not. That hidden identity is later defeated by Fubuki, though it's a lot more questionable if that counts as unassisted. The other Dragon Class monsters are defeated either by combined efforts in a big battle, or by Saitama or Garou, or technically survived, or some combination thereof.