It's not a requirement to kill monsters
Astera advances when you're on expedition for 3 minutes, a mission for 3 minutes or have a quest complete.
Since you should be able to kill Jagras in under 3 minutes (with high rank gear) make a determination of if you wanna do quick missions or just sit idle in expedition (Ancient Forest) for 3 minutes, then use world map to fly to Wildspire Waste for another 3 minutes.
The strategy is up to you, but just note that it can get boring kinda quick.
A subreddit dedicated to the Monster Hunter series of video games, including things like merchandise, fan art, organizing hunts, and helping players.