Many people probably know this but yeah you can transfer any epic or above monster from HI. Since you can only send one i recommend sending a legendary, if you don’t have one (i didnt) do this.
Start a new game and go through the tutorial, you start with one egg and 250 gold. Use all that to get some eggs (Go to the Items section for an egg for 1 gold and one for 150). This means 3 rolls per new save. If you dont get a legendary, delete save and retry. I did this until i got a legendary (Gigarock) and transfered it. I was having trouble on a boss but when i added this monster to my team i 8-0’d that boss. Definitely don’t waste your transfer on anything below legendary, go for the best!