Haven't seen this tip yet, so give it a shot. It's worked 5 out of 5 times for me so far. This is a tip that coincides with the Harpoon Net trick.

1) single out the Managarmr you want while mounted on a daily strong flyer. ie: wyverns, snow owls, etc.

2) reveal yourself to the Managarmr by dismounting and remounting until it begins to attack, which isn't a lot of damage.

3) once the Managarmr begins to use its divebomb attacks, fly upwards and land on one of the large corrupted trees that resemble a lotus.

4) Stay mounted with the Harpoon gun equipped and loaded. Once the Managarmr dive-bombs, dismount and trap.

5) Tranq and Tame.

This not only allows you a better opportunity to net it, but also to keep it away from other Dino's while taming.