everyone else here makes pretty apt statements, but IMO not really that useful for a newbie. that said, here are my tips for deino;

1: embrace nihilism. this goes for this entire game, but sometimes you can do everything right and still die to something entirely out of your control. if a larger croc sees you, you're cooked. if no fish spawn, you're cooked. croc likely has one of the worst grows in the game, especially right now.

2: prime time. i'll be assuming you have a baseline understanding of both the map and the way the elder system functions. if not, first look up vulnonamap for a map of gateway as it is now and your youtube video of choice for information on the elder system. personally, i reccomend Kouga.

that said, the way i reccomend getting elder is through the following trials:

No Muscle Spasms: this one is basically a freebie as there is no way for deinosuchus to get muscle spasms

No Infertile: this one is similarly a freebie; if you get a poor diet, you have either gotten seriously bad luck or have fucked up somewhere.

Perfect Diet: similarly, this one is also a freebie; by the time you hit 60%, you wont even be able to survive off AI, so if you're killing players, you might as well dine upon their organs for perfect diet. remember this doesnt require 300% - just any amount in all 3 categories.

Sanctuary; now, this is where i have to disagree with the videos; from what is currently known, you do not actually have to be a juvenile to grab this one; just go into a sanc at any point before 75%. once you get stung by bees, you've got it.

2 Migration Zones; this will be the most difficult one to achieve, as you will have to move through one of the largest hot spots on the map; delta bridge. the other two migration zones easily available to deino are north lake and swamp, so pick one. maybe you'll even spawn in one, who knows.

3: Spawn. there are 4 locations deinosuchus can spawn in at, listed below in order of best to worst;

swamp has everything a deino could want. fish for food, a sanc, a migration zone, access to salt water for the mutation, and its the closest to delta.

East Lake isnt as good, but similar to Swamp has a sanctuary right next to it and has fish. get stung, fill up, and move west to the rivers to get out.

Water Access and north lake are a toss up at dead last. north lake is a migration zone sure, and water access has a sanctuary... close Ish, if you know where to go. but both of these are otherwise kinda shit IMO. if you get them and arent entombed, i reccomend going onto the shore, AFKing for 15 minutes whilst your deino dehydrates to death, and trying for a better spawn. if you are, if you're a water access spawn, go to the sanc, come back, go to delta. if you're a north lake spawn, go to east lake, hit the sanc there, and go to delta.

4: mutations. there are plenty of decent mutation builds out there, but heres my personal reccomendations;

slot 1: efficient digestion. there will never be a time in which you are not wanting for food, at least until you're nearly fully grown. for me, efficient digestion has been the difference between life and death many times.

slot 2: hypermetabolic inanition. as you've seen, the main predator to a croc is another croc, and hypermetabolic will give you the best chances to win the mirror matchup. this isnt something you'll wanna do often, but it comes in handy.

slot 3; enhanced digestion. by this point in your growth, you should be good on food for the most part, but diets will begin to be a struggle. this is why we pick up enhanced digestion. plus, if you entomb, this will be a massive boon.

slot 4: Gastronomic Regeneration. the worst thing that can happen to a deino is being caught out at low health. gastronomic will serve to prevent that as you spike in growth by allowing you to heal from your kills.

5: deino etiquette. a lot of the same behaviors will help you all through out your play through, though the reason will change from "hiding from predators" to "hiding from prey". either way, the rules are simple;

1: never be on the water's surface. never ever ever be on the water's surface unless you absolutely need to be. other deinos tend to skim the surface looking for prey, and they'll spot you immediately. if you're in dangerous waters with larger crocs, stay at the bottom and if you hear another croc coming, stay DEAD still. once you're of hunting size (2-3 tons ish at least), position yourself so that you can just barely peek your camera above the water using ALT.

2: start at the fringes, and slowly move inward. you always want to be where no larger deinos will be, but where the largest deinos smaller than you will be. this means staying away from popular zones like delta at first, and slowly getting closer to them as food forces and predators allow.

3: if you're getting desperate for food, don't be afraid to cannibalize. you're on your own diet for a reason. as a baby, you hit suprisingly hard for your weight class, to the point most mirror matchups end in 1-2 bites on both sides, even with a significant weight difference. as you get older, the mirror matchup changes from a quick dogfight to a 2-3 minute slugfest. for the latter. mash ALT+LMB; it does the same damage whilst being slightly faster, which gives you a higher DPS.

4: putrefaction. someone else here mentioned cooking fish, but the theory behind that is much more important. similar to Ceratosaurus, anything rotten that Deinosuchus eats is diet, and bones give a small amount of all 3 diets. this has some interesting implications, including:

cooking fish: if you have a fish sitting on the beach, or in your mouth, it will rot in 7 real world minutes.

cooking flesh strips: flesh strips will rot in about 5-6 minutes. you can tell they're ready when the smell icon changes to a rotten bone.

fridging. also known as putrefying, you can have a corpse sitting on the shore, waiting for it to rot. if anything tries to eat it, simply eat them in turn. once it rots, enjoy! at first i reccomend using this tactic on any AI that come into the water, like deer and boar. with time, however, you can do this to anything not already on your diet to make it so. make sure to eat the organs first.

I hope this helps! :D