In your Game.ini put this on the bottom and save:

DinoSpawnWeightMultipliers=(DinoNameTag="BossMosa",OverrideSpawnLimitPercentage=False,SpawnLimitPercentage=4.0,SpawnWeightMultiplier=8.0)

The numbers can be adjusted until he spawns as much as you prefer. You'll have to do a dino wipe each time you change it though.

To the above message.
SpawnWeightMultiplier=8.0
This is the overall SpawnWeight, when this is higher, creatures in the affected SpawnEntry will be spawned more often. Note that this is a multiplier, so when the base value is low (eg. 0.01) then you'll want to use bigger numbers to get it up.
SpawnLimitPercentage=4.0
This is the actual limit of the creature in affected SpawnEntry. If OverrideSpawnLimitPercentage is False then this is ignored. Be warned that this value might cause an overspawn, so try experimenting with SpawnWeight first.

Originally posted by Wolfae:

In your Game.ini put this on the bottom and save:

DinoSpawnWeightMultipliers=(DinoNameTag="BossMosa",OverrideSpawnLimitPercentage=False,SpawnLimitPercentage=4.0,SpawnWeightMultiplier=8.0)

The numbers can be adjusted until he spawns as much as you prefer. You'll have to do a dino wipe each time you change it though.

But mosasaur still does not spawn(

in the past we had to spawn it in to kill it .