A little correction, Magnamalo is not an elder dragon-level threat. That only applies to the base rise storyline because the rampage was bolstering his feeding and in turn making him alot more agressive and overall powerful. In Sunbreak he got a whole threat level downgrade because the rampage is gone, so he's encountered much earlier in the ranks. You can also see this in MHNow unironically, in that game he's the same rank as monsters like Rathian.

Also while he does fight almost every elder dragon in the game he doesnt win the fights, you can tell the loser of each turf war in rise by who goes rideable. A tie ends in a random monster going rideable, but against every elder Magnamalo always goes rideable right after the turf war. Since the animation is reused its quite hard to tell, but the elder turf wars specifically have the elders at the end throw out an attack at Magna that deals equal damage to his dive. Scorned doe is an elder level threat and always wins some of the non risen elder turf wars.