This guide explains how to beat Magnamalo using hunting horn squad.
Guide to Kill Magnamalo
Use hunting horn squad.
Jump with the silk bugs while near Magnamalo when you have the fire blight status.
Ride a rathian over to him and hit him as much as possible and end it with the special attack.
Then ride him and take him to mizutsune so you can ride that and attack him with that as well, focus on the parts of his body glowing from fire.
Also you can use 2 GL (Gun Lance) and 2 LS (Long Sword).
You must work as a team and keep triping him and traping him and you’ve made use of the blights. It also helps to play in a premade party then with randoms.