.... I'm disappointed that you lack a picture. That needs to be fixed immediately. Because pictures make everything better.
There are lots of tier 3-4 classes that could do what they do best without being threatned too much, but I still wonder if the Sentai is too good at some times... mainly because of all of the options. You could have 5 people in a party all Sentai and each be different from eacho-oh...
All and all, I like it. Though I wonder what other options for Prestige Classes could be taken for the Sentai, beside the obvious need for a prestige class that enables a giant robot or some kind to be controlled by the Sentai... A lot of potential ideas are somewhat covered in the various abilities...