I have done this by selecting "Gajalaka Outbreak" in the gathering hub which counts towards this trophy.
I took the HBG (Heavy Bowgun) and when you are in the arena switch to Normal 3 ammo. It takes 2 shots to kill one gajalaka. Trap the roaming baroth with the two traps you have in your inventory so you can attack the gajalakas without that much interruption. My best time was 1min09sec. It does noch take very long to reach 50 arena quests done. It is boring yes, but you can get this trophy out of your way like this. Happy huntuing!