Large monsters include every monster with an icon on the minimap, and every monster you can gain Research Points for, from the Great Jagras all the way to Xeno’Jiiva… basically every Fanged Wyvern, Flying Wyvern, Brute Wyvern, Piscine Wyvern and Elder Dragon in your Hunter’s Notes. To earn this trophy/achievement, you must slay one hundred monsters, which - unless you manage to get through the story without hunting monsters repeatedly (how are you getting enough materials to keep your gear up to snuff otherwise?) - you should make significant progress towards this just playing the game normally. If not, you’ll certainly get there if you indulge in any significant post/late game grinding. Failing that, you can just repeatedly hunt the Great Jagras to boost your kills, if you only care about earning the trophy/achievement, but you’re better served focusing on more worthy prey for the trophy/achievement "Established Hunter".