The Sentry Guardian has the ability to float, can nearly one-hit kill on contact with the player, and spawns Sentries. Sentry Guardians will spawn several of those when hit. Attacking a Sentry Guardian will make it's highlights turn red. This makes a Sentry Guardian a difficult opponent, especially since Dart Shooters have no effect on it.
Attacking the Sentry Guardian, falling back, luring the Sentries away and going to attack the Sentry Guardian another time is, possibly, the best technique against this mini-boss as one can avoid taking damage and still kill the mini-boss rather quickly.
It is also recommended to bring Gummy Swets if one decides to be aggressive against this boss, or fails to juke Sentry attacks. If you don't have Gravitite Armor, bring several Cold Aerclouds, as those will prevent damage before falling if placed quickly before impact. However, that requires split-second timing and is difficult to pull off.
It is also possible to just build a 2x1 shelter to counter against the Sentries as their explosions do not deal much damage at all. Hiding in the shelter will ensure you will not fly away but however it will take much more longer to kill Guardians as they most of the time do not come close enough for you to hit them with a sword.
If you can't win, exit and come back in with a few stacks of blocks. Find the Guardian and then try to cage it in with blocks. Once you've done the first layer, pillar up until you can completely cage him and prevent him from moving anywhere at all. After that, break a hole and just hit away. He will not be able to hit you unless you break too many blocks or move too closely to him and the Sentries should be trapped too. Just punch him to death if you do not wish to waste your weapon. Note that after it is done, do NOT enter the cage for the tokens or you will die.
The Sentry Guardian was created by someone on the Aether community. Named as "Aether Golem" or "Aerogolem", it was meant to be a neutral mob instead of a mini-boss.
Issues relating to Sentry Guardian mob are maintained on the bug tracker. Report issues for Sentry Guardian there.