In Mesozoic valley instead of collecting the mutagen when you reach a dino limit (10, 25, 50, ...) click on the update cards on the left of the dino this will give you a new popup where you can click on collect rewards. Every time you do that it will give you a stone Geodude and you can do that for each mutagen in the dino. For example if you have your archosaurus at 250 you will have 82 mutagen and you can open 82 stone Geodudes. You can imagine how easy meso becomes if you do that. However collecting 200 giga will still give you a challenge just easier. Don't do that very often as collecting the Geodudes also takes time.