Leon had certainly taken an elixir, but it was the Bloodthirsty Elixir, which didn't impact his strength.
He could match Angmog's strength as a mature Red Dragon because he had grown considerably over the years, evolving from a teenage dragon to a youth dragon, advancing from a large size to an enormous one. While he was still no match for Angmog, the gap had closed significantly—and there was the added boost of Bull's Strength from the Cloud Giant.
In fact, if Angmog hadn't progressed to Blood Scale Frenzy—which increased both strength and constitution during a berserk state—Leon might have even been able to overwhelm him instead.
But there's no "if" in this world. The reality was that Angmog had advanced to Blood Scale Frenzy. In terms of strength, he was still superior to Leon. He just couldn't toss Leon around as effortlessly as he once had. Now it would inevitably come down to a prolonged standoff, wearing down Leon's energy bit by bit, then finishing in one decisive push.
