Basically, on the sword tree I believe there should be a "shield branch" in which you could use a shield using right mouse button to block arrows, swords, and the bombs. Obviously you should not be able to attack while using shield, and you would be defenseless from behind.
Despite what most people think, the arrow deflection arrow upgrade isn't that good.
It's mostly best for beginners, but in reality, the jetpack and the kick have so much more potential.
Once people start to master the game and realise that it isn't that good in reality, more people will start to go for the other upgrades, like the jetpack, bow and kick.
I love the jetpack, it is SUPER OP, in the case where you can just hit and run enemies, so you dice them up all while getting out unscathed. I also really love the bow, but my aim is terrible and half the time I am forced to use the sword because my arm got lopped off. I still really love the deflection though, cause it has saved my life and gotten me kills more times than I can count. But that's just my two-sense on the subject :P
The jetpack isn't OP.
Neither is any of the weapons.
The Jetpack is made for fast manouvers. If you are quick at moving and have good reflexes, and know how to use it, then yes, the jetpack is OP. Same as all of the other upgrades.
The bow is OP is you can aim properly, simple as that.
Deflection isn't that good. It leaves you vulnerable in later levels, as archers are positioned in better and better places.
The deflection is easy to use, and better for earlier stages. That's why lots of people are flocking towards it.
Though I will agree, it can be useful in lots of situations.
(Then again, I'm no master of this game myself. I just got it 3 days ago!)
You could make it so it was a bit down the tree, and some other people have ideas to nerf it.
Maybe a boss with a sheild? Its probably not going to be spider-tron 7000 tho.. But maybe... But a sheild boss would be cool to have in the game, espeacially for later levels of endless mode