You may have noticed that unlike any other steal and hatch tame, magmasaurs can deal damage to you while you ride your mount, SO THIS IS HOW ITS DONE!! You get a ferox lvl 20 (higher if u can but dont name it... you'll see), then you lvl him in health, 500 in small form is ok, after that you have 2 ways:

1)KAMIKAZE(bloodstalker needed or some skill woth grappling hook); you transform your ferox and whistle him to riot right into the nest, the point of this is that all magmas use their magma bombs against him, so you can pick up your eggs and get out of there (its called kamikaze, dont make me write it)

2) I'LL TAKE YOU ALL ON; transform him, whistle attack, riot 1vs4-5-6, and then you charge with your highest dps dinos, lava doesnt do as much damage as on other maps, plus you can stay at the edge of the lava and you'd be fine, i used 2 rexes mated boosted w 7k health and 500 mele and i got out with everything alive and 6 magma eggs, you can use allo packs or spinos too. Extra tips? for

if you are on fjordur and have a desmodus then you can go up and make it Hang on the cave's ceiling while shooting at magmasaurs

They will focus you, just go down and make sure to drag them into another place of the cave so the egg you wanna steal is free of any dangers

If there are onycs or any other npc near but you will wanna flee fast before the magmas catch you simply put the desmodus on passive and quickly pick the egg

NOTE: this can be done With any flyer that lets you hold weapons while riden (if you have the setting on)

I recommend desmodus because of his invisibility and abilty to be cave-friendly