I have heard many people say that the game is too hard because people constantly steal all of their gold in the monster arena. I made a strategy for working against this because I was having the same problem at times.
The first thing to remember is that gold in your habitats is safe. It doesn't become something that other players can steal from you until you collect it.
What I started to do was collect about 30k gold and spend all of my food. Then, wait to be attacked. If you are a low level and your monster arena team is weak, you might need less gold to be a target for someone. And, vice versa, if you have a strong team you might need to have more gold to interest someone to attack you. The goal is to get enough gold to make them want to attack and activate a shield on you, without losing much gold or food. Then after you are attacked and the shield is activated, you can collect the rest of your habitats. I also recommend upgrading your habitats so they hold more maximum gold. So all you need to do is keep your trophies down
If you use this strategy just make sure you have used your gold and food before your shield runs out, and don't attack anyone in the arena and lose your shield.
Another thing that you can do if you want to still fight in the arena and not use a shield, is to purchase habitats with all your gold and then put them in storage. For example, if you collect 227k gold you can purchase a dark habitat and put it in storage. Do this several times and when you need gold you can sell them all at once from storage and then buy whatever you needed to save for. The downside to this is that you need to keep below your maximum amount of habitats (at least 1) so that you can keep building them and storing them. Another issues is that you only get half of the gold back from selling them. Still, it's a workable method for saving up enough gold for something that you are having trouble purchasing. If you can save no gold for a legend habitat, and store 45 dark habitas, you can sell them all at once and have the 5m gold to purchase a legendary habitat.
Another important thing to remember is that you don't lose gold or food from attacks until you refresh the page or leave your base. So if you can collect 500k gold in one day and leave the game running on your computer, you would need only 40 dark habitats. I have done this twice for myself, but I used a combination of dark, light, and water habitats and saved around 1.5m gold in one day, and then sold the habitats to afford the legendary habitat. MAKE SURE YOU HAVE AN ACTIVE SHIELD WHEN YOU CASH IN YOUR HABITATS SO NOBODY FROM THE ARENA STEALS YOUR PROGRESS.
I just made this picture to show the 2 legendary habitats, but while I was putting it together from 3 different screenshots in MS Paint :P I started a new breeding and it will be a legend. I was getting angry because after breeding Rabidex + Rabidex about 20 times in a row, I had nothing better than a Rabidex to show for it. Then finally I got the Thorder egg, and now - around 5.5 hours later - I started another breeding and got another legend. There was just one monster in between the two legends, a Rhynex. Until two days ago I was pretty angry with the breeding, as I had tried many times but never had gotten a legend until then. The same image can be found at full resolution here: full res