Dying a lot - Underleveled?

[Alternatewarning]

I keep going back to this game and then dropping it for months at a time because I find most of the battles so dang hard.  I've been completing every single mission and re-completing the story missions so I don't think I'm under leveled?  I just don't understand if I'm doing something wrong or why no one else seems to be struggling with this game.  The characters I'm using are all around level 50 and I'm on normal difficulty.  I'm working on the mission "Destroy the Crystal" which is chapter 7.  But it's not just that mission, there are other rank B side missions that I can't pass because the tetris enemies kill my entire party by blowing them all up from halfway across the map on turn 2.  I tried to grind out some levels but it's taking forever and doesn't seem to help my survival at all.

[kreatur_84]

You have to level them up anyways.yeah I think you are underleveled.

You should also Use the Element. If one enemie is weak against your Element, you do a lots more damage. 

You can also swap units if you got enough points, so you can change Element