It was a really bad design.That sounds like bad game design to be honest. Based mainly from my experience with Zelda games but Zelda likes to have these puzzle bosses during the N64/Gamecube era where bosses required using the item you obtained in the dungeon in some way to get the boss vulnerable. I think one thing Zelda does well in this regard is that it "teaches" the player how to use the item and the method of getting the boss to expose its weakpoint uses a method similar to how that item is used. Also helps that Zelda has a history of having "puzzle" bosses which means the player is prepared with that mindset while Sonic traditionally is a bit more straight-forward (please correct me if Im wrong in that regard. Not saying sonic bosses are easy but they require more knowing where Robotnik's mech's hurt boxes and attacks are and timing your jumps and hits to avoid attacks and damage robotnik).
Also, putting the target behind the boss is a dick move, not going to lie. Im assuming the boss turns towards you while fighting which means getting behind them is already difficult. Were there enemies that required you to hit them from behind? Not having to change targets until now also seems like poor game design to suddenly introduce a mechanic you havent used until then.
The older Sonic games were find the hit box, for the most part. The newer 3D ones were a mic of that, along with sometimes waiting through a sequence, mostly just staying alive until that hit box opened up.
Some have had puzzle like ideas where you had to figure out what to do, but Sonic Frontiers is kinda trying to be a Zelda game, but instead of new items you get, instead, you learn abilities for him.
Some are up to the player to ever use at all, like combos while fighting. You could go through the whole base game without even needing to change targets.
So heres a screen cap, and I added thr yellow arrows.
The game seems to ALWAYS want to target the head, but you have to hit the pink cord part first. And then the other target once life is drained. AND THEN hold triangle, which the game also never tells you to do, at all.
It could be done fighting normal enemies as part of a combo hit, but this was the only place where you HAD to do it.
Hopefully Sega learns from this.
I see the screen has tips. I realize, I had it turned off for my playthrough.