Every Boss will automatically counterattack (aka. Retaliate) after some time. This is because of a specific value that is called "Revenge Value".
No matter how hard you lock the 'Lingering Will' boss into a combo, he will eventually counterattack out of nowhere. This counterattack can actually be calculated, which makes it very easy to dodge.
Every attack you do raises his Revenge Value. After going above a specific Value, he will automatically retaliate.
A good rule of thumb: Every normal attack you do raises this value by 1, a Guard Break (stabbing thrust) finisher will raise it by 3 and the Explosion finisher (The magic balls around you) raises this value by 6.
An Example: Data Roxas has a Revenge Value of 9.375. This means: If you do a 3 hit combo (2 normal attacks and a Finisher) with a Guard Break finish at the end, he will not counterattack. His Revenge Value is currently at 5/9.3, which means he is still 'vulnerable' and you are still in your attack-rotation. If you do your second finisher, an Explosion, then it will be at 11/9.3 and he will counterattack, ending your attack-rotation.
Now, let's equip 2 Combo Plus' abilities. This raises the default combo hits to 4 (instead of 2). The Revenge Value, after 4 hits and a Guard Break finisher at the end, is now at 7/9.3 instead of 5/9.3. This means that, even though we did 2 more normal hits, we still didn't make him counterattack. We did more damage in nearly the same time and without doing anything special. Because no matter if we equip the Combo Plus'es or not, he will counterattack after the Explosion Finisher. The whole fight is over more quickly and there is barely any change in strategy, even though you hit him an extra 2 times and dealt a lot more damage than before. This Optimization was possible by looking at the Revenge Value (or by simply testing it out).
Visit My Combo Plus article if you want to see the damage comparison and .gif of the Combo Plus Roxas fight mentioned above.
The Revenge Value is only available if the enemy is in an 'exhausted state'. Attacking an enemy while he currently attacks will most of the time not raise the value. As a rule of thumb: Revenge Value only exists and raises if the enemy staggers from your attack.
BUT: The attack that you reflect onto your enemy to make him stagger and weak (which makes him vulnerable) sometimes already adds a specific value to the Revenge Value. Example: The Lighning-Cannonball that the Lingering Will shoots. His revenge value will start at 4 and not 0 if you reflect it back to him and he gets sent flying. This only works if the attack that you reflect knocks the enemy out of his combo and makes him vulnerable to your attacks. This won't work if you simply use the spell "Reflect" to reflect his attacks back at him. Example: Reflecting the "Dash Attack" of Lingering Will won't add any Revenge Value, because he still attacks afterwards. Reflecting the Energy Ball he shoots with Guard back into him will knock him into the air and make him vulnerable against your attacks, that's why the Energy Ball raises his Revenge Value.
You should never raise the Revenge Value over the edge while attacking a boss close up. Most of the time they will instantly counterattack you, and dodging out of a normal combo hit is near impossible. Try to exceed the Revenge Value with a Combo Finisher, because then your enemy will be further away from you and you can easily dodge his attacks this way.
There are also specific attacks that do NOT add Revenge Value. These attacks can be used to deal a lot of damage to an enemy without them counterattacking. One of these abilities is "Ars Arcanum", which is available while in Limit Form. Only the last triangle hit of this ability adds Revenge Value, every other hit does not. This means: Instead of doing a normal combo and him counterattacking, you could do: Ars Arcanum, a single hit to keep him staggered, another Ars Arcanum and then a normal combo with a Finisher. This will add around the same Revenge Value as a normal combo.
Another ability that doesn't add Revenge Value is Final Form Firaga. Using Firaga while in Final Form doesn't add any Revenge Value, only the Firaga-Finisher does. This means that, if you want to do a LOT of damage in a short amount of time, use Final Form Firaga. For extra damage equip Axel's Fire Keyblade in the Final Form Keyblade slot.
EDIT: These 2 Strategies mentioned above (Final Form Firaga and Ars Arcanum) work 100% in the PS2 version of KH2FM. I tested them against Lingering Will in the PS4 version, and I had some errors and sudden counterattacks here and there, so it seems like they tried to fix it in the newer versions. They still work most of the time though.
Lingering Will has a Revenge Value of 25, Most Data fights have 9.375, and most other bosses have around 12.5. I don't know every value of every boss or every ability, so please use the internet for an exact research.