This guide will provide you with the fastest and free method to get Robo Tito in Monster Legends Tales Challenge.
To get the Monster Legend, you have to use 46 gems, only 20 cells, 12 hours into the event, and then 41 gems on the 30 cells, 1 day, and 16 hours in the event. Then 76 gems on the 50 cells, 1 day and 12 hours into the event.
You have to make sure to do all of your collections in between those times because that is how you’re going to get it as low as possible.
You don’t have to use gems on any of these and you do definitely want to get the tales key. You have to bring it into the chest shop and you can turn tokens into cells, and then, you can use 200 of those to rank up the monster. 
Then you just need to watch ads in the chest shop and open up the daily chest which will give you a token. You can also do the time challenges to collect gold, and feed monsters. Also, when you do your daily mission, you will get one token.
In the dungeon, you can get 15 tokens, and then during breeding events, you will also get one token for getting the monster out of the breeding van.