ok i watched the final boss of bbs and i wondered ur fighting terranort while playing as terras armor wats going onthere?

you should go check youtube for what happens.

in fact choose a video with english subs

The reason Terranort is fighting Lingering sentiment is because master xehanort took over Terra's body. Then Terra's memories took shelter in the armor thus bringing the creation of Lingering Sentiment. The simple reason why lingering sentiment is fighting Terranort is because lingering sentiment remembers the rage Terra felt for Xehanort before he took over Terra's body.

kingdomhearts12 pretty much summed it up

ok i watched the final boss of bbs and i wondered ur fighting terranort while playing as terras armor wats going onthere?

 

Master Xehanort had just extracted his heart and sent it into Terra's body, which he uses to take control of. Terra's will was so strong, that his lingering sentiments were able to animate the armor he left behind, thus creating the Lingering Sentiment. They continue the battle to show that Terra will never give up, even though he lost his very self.

 

And because I always have to repeat myself, the Lingering Sentiment is very literally Terra's lingering sentiments. Not his memories, nor his heart, nor his soul, but his lingering sentiments.