The Guardian (ガーディアン) is the fourth mini-boss of Sonic & Knuckles, fought at the end of Sandopolis Zone Act 1.
A rather unique fight, the Guardian is a stone statue that doesn't even look like a robot, and has no weapons, but jumps toward the player. The player can knock it back into its shell form by jumping and hitting its head, but they cannot hurt it this way. To defeat it, they must lure it to the left and get it to jump in the quicksand pit. This is the only way to defeat it. The most efficient method of doing so is to stand on the edge of the pit until it gets close, then jump into the pit and jump continuously to avoid sinking. The player should make sure they're far enough away so that it doesn't jump on them (although it can't crush them if it does jump on them; it merely causes damage). It will jump into the quicksand and blow up.
It is also possible to beat this mid-boss by standing on the right side of it and hitting it on the head repeatedly. Though this avoids the possibility of it jumping on the player when it reaches the pit, it also takes longer for it to do so.
This opponent can prove unexpectedly difficult to defeat with Super Tails, as his Super Flickies are just as likely to knock the Guardian away from the quicksand as towards the pit.
When playing as Sonic and Tails together, Tails can either help or hinder the player. If the player chooses the first method of standing in the pit to lure it to them, Tails could knock it back towards the right by mistake. If the player decides to knock the Guardian all the way back, they can get Tails to stand in front of them in such a way that when they jump and touch , he does the same and (hopefully) hits the Guardian on the head. This latter method is a good idea if they have a lot of Rings (that you wish to keep) but no Shield.