So I want to throw a nice big mini boss at my players while they are in a dungeon. That said, decided to go with a bit of a Legend of Zelda themed one, and have it be a giant frog that spawns little babies to attack them. There are 6 players; cleric, sorcerer, witch, rogue, ranger, and chevalier with an average level of 8. So just a normal giant frog with the ability to spawn things is out of the question.

I've never advanced a monster quiet this far, I am fairly new to DMing, so I have just been using the premade stats from the books. But for this I want it to be either large or huge. Leaning more towards huge personally. I want it to be challenging but not overwhelming, enough for the players to remember it as something they had to work at to beat.

For the spawning ability I'm thinking of making it take a full round action and spawn a 1d4 of either poison frogs, or giant frogs, and have them join the battle. Maybe a rule that if the players do a large amount of damage some extra ones shake off? Maybe a 1d3-1 kind of thing?

Any thoughts, or tips on how to make this thing?