While we don't know every way in which the people of Thedas, save for ourselve of course, have devised stratgies to slay High Dragons we do know of a few.
For example, the Hivernal High Dragon was hunted routinely for its hide and meat by early Orlesian settlets to stave off winter and hunger. They apparently did so in groups that numbered in the dozens. Which could means they would have some groups to tire the particular breed out or chase it into larger groups to finish it off or however else Hunting parties use tactics to kill prey.
The Pentaghast seemed to have killed dragons in numerous ways. It's not clear if they used multiple hunting parties or fought alone though it's likely they did both. It's possible Cassandra's brother, Anthony, was one of these lone dragon hunters but given that we don't know how he went about slaying dragons it's hard to say.
We do know based on what Cassandra tells us that before they'd become deformed and insane that some Pentaghast would become Reavers to use the power to kill dragons more effectively which in turn would have left them with more dragon blood to use.
The Baroness kind of sort of killed The Queen of Blackmarsh when she was still human, but since she really only banished it's essence to the Fade and then had to spread its bones around to keep it from coming back. Given that it's not a conventional method being or otherwise conventional magic that would have kept QoB dead the first time I guess that doesn't quite count.
We can assume the Qunari have killed or at the very least would have plans to kill High Dragons given that based on what Iron Bull says we can infer that the Qun believe they have to be destroyed since they're considered savage and chaotic and while its not known how exactly they would go about doing this it can be inferred from the captured and mutilated Ataashi that it wouldn't take them much to figure out how to accomplish this with a large group at hand.