Maybe Shulk doesn't move because he is bound? I think the game shows that with a couple of rotating rings above the head of a character. So a spider bounds Shulk, the targeted enemy is out of Shulk's reach, therefore he doesn't attack. Since he doesn't attack, he can't fill up the Monado gauge. Since the Monado gauge isn't filled, he can't activate Monado Shield to protect you from the boss's most devastating attack. And that's why you lose.

But evidently you don't know how to give commands. If Shulk attacks the little guys, then that's because you didn't tell him to focus on the boss. If you use a Classic Controller, you have to hold down the ZR button and press a direction on the d-pad; pressing up makes your party members attack the enemy that you have targeted.

So much for that, but in Monado-focused battles it's for the best to play as Shulk yourself. That way it's much easier to control the Monado gauge and issue the appropriate action at the right time. Playing as Reyn is pretty boring because it's all about aggroing the enemy; with Shulk you can move around the enemy to deal additional damage with the correct positioning. If you still can't beat the boss, then you have to level up some more. Either through conventional grinding or by solving sidequests that are handed out by named NPCs; these usually net a good amount of EXP, plus various equipment.

By the way, most JRPGs consist of running around, fighting monsters and watching/reading story.

I would recommend playing as Shulk, while Shulk is attacking the big spider, have Reyn attack the small ones. When you get a vision that can be blocked by shield, use it. Make sure you use Stream Edge often, to ensure you hit multiple enemies.

http://xenoblade.wikia.com/wiki/Stream_Edge

Shulk learns it at level 5. Not only does it hit multiple enemies, but it also builds up the monado gauge. 

That sounds like you are locked on to a little spider that is behind the boss without realizing it; Monado Shield has no influence on what you can attack. If you are close enough to the targeted enemy, Shulk will swing his sword. That's just how it works. Bosses and special monsters have green borders on both sides of their information box. If you are facing the boss, but the box doesn't have such green borders, then you are definitely locked on to the wrong enemy.

If you aren't doing sidequests, you will have to fight a lot more battles. Seems like you are stuck between a rock and a hard place then.