If there was any card out there that could make Retreat to Kazandu playable in a Hardened Scales deck, it would be this guy. With Hardened Scales already giving you an extra +1/+1 counter, Return to Kazandu then allows you to get yet another +1/+1 counter for each land drop. It’s not unrealistic to have Undergrowth Champion become a 4/4, then a 6/6, then an 8/8 very quickly, and damage based removal doesn’t do much to this guy - hence his mythic rare status.
This guy is a four-of staple in Hardened Scales decks. Even if Constructed doesn’t really find room for Retreat to Kazandu (a 3-drop enchantment ends up cutting down on the creatures you could be playing instead), Hardened Scales alone will make Undergrowth Champion quite a force to be reckoned with. Do keep in mind that any time he’s blocked, and combat damage is dealt, he’ll lose a +1/+1 counter. He may not be quite as busted as he at first looks, but he’s definitely a constructed playable card that will make a super good friend to Servant of the Scale, Avatar of the Resolute, and other +1/+1 counter happy creatures.