Does the A press to "X is attacking Y" need to be on-beat to initiate a combo attack?

Kinda. After completing the game and trying to combo as much as possible (to the point of saving a state before every single attack and loading it back if I didn't get enough combo) I think I have the right mental model for how it works.

Basically, the answer is two-fold:

• Technically, no.
• Practically, yes. Kinda.

I'm going to show a rough timeline like so:

♫: ---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---

This line shows the "hidden heartbeat" of a generic enemy, with a little bit of tolerance to the sides. If we add to the timeline a track showing when you'd press A, this is what would happen in an ideal world: you'd get each beat exactly right.

♫: ---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---
A: ----A-----A-----A-----A-----A-----A-----A-----A-----A-----A-----A-----A----

...but even if you were slightly off, the game would still accept it:

♫: ---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---
A: ----A----A-------A----A----A------A------A-----A----A-----A------A----A----

However if you miss a (♫) entirely, your combo is over and control goes over to the next character. I'll show combo-ending A presses with a ×.

♫: ---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---
A: ----A------A----A--×--- 3 hit combo
A: ----A----A----×------×- 2 hit combo

Now, what you would do in practice is get the "Lucas attacks!" prompt, start tapping with the beat on the controller off-button, then start pressing A away. The question is about that first A press.

The game does not check if that A press is on beat, or else you wouldn't be able to get combos going accidentally with a turbo controller:

♫: ---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---
A: -------A-A-×-×-×-×-×---  accidental 2 hit combo!

This actually happens fairly frequently (of course, the faster the tempo is the better) and, on some of the harder songs, it's probably the most reliable way to get a little extra damage through. Given the way the rolling health counter works, this also means you might be able to avoid taking a little damage.

However, long combos are typically worth it. While diminishing returns are in place, a 16 hit combo can normally deal about 2.5× as much damage as a single attack. (Finally, if you're a dirty scumbag save-state cheater like me, you can use the length of the combo to affect the RNG and enemy attack patterns or successful use of items or techniques. This can be handy on the tougher battles!)

So, as I have said these two series of A presses are just as good:

♫: ---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---
A: ----A-----A-----A-----A-----A-----A-----A-----A-----A-----A-----A-----A----
A: --------A-A-----A-----A-----A-----A-----A-----A-----A-----A-----A-----A----

...but if you are a human being, rather than a tool assisted speed runner, you'll have a far easier time nailing the first line of beats. In other words, if you start your sequence of beats off, you'll have quite the hard time adjusting it going forward.

So, yes, you do want that first press to be on-beat, even though it needn't be.

The devil here, however, is in the detail. If you're slightly early with your first A press, you basically won't be able to combo at all!

♫: ---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---(♫)---
A: --A-------×-----×-----×-----×-----×-----×-----×-----×-----×-----×-----×----

No matter how on time you would've been with each beat on the line, you accidentally missed the beat right after the prompt and suddenly you can't combo any more. This is not a good place to be and, if you're not cheating with save states, you probably need to try and stop for a moment and listen to the tune so you can readjust your timing before attempting another combo. Putting the enemy to sleep, as always, helps.

Finally, if the whole combo system bores you to death and you still want to deal awesome damage to your enemy, the ever so helpful starmen.net has the Gameshark v1 cheat code just for you.

A986DFE1 8532B3F6
7750E1A0 3FACA8E8
368FBAB5 4691351A

With this cheat code, you will simply need to press A 17 times every time one of your characters attacks. A turbo controller is advised for maximum cheatinghood.