Is there any block that can't be shoot through but skeletons don't know it?

I modified my mob grinder in such way that it now collects almost dead monsters in a small 2x2 area. From this area, I allow skeletons to shoot at me. As they shoot, they hit creepers and produce nice amount of music discs.

The problem is that huge portion of arrows still makes it through. I tried to reduce the damage with water, but even 3 blocks of water still aren't enough:

My question therefore is: Can I make such set-up that skeletons will fire me but will never hit me?

Solution 1:

You could set up a small redstone contraption in order to only expose the skeleton to you if there is a creeper there.

If you want, I can make a prototype.

Also, you don't have to risk yourself: They retaliate from attacks, and snowmen attack skeletons, so they'll attack snowmen (after it attacks them)

Solution 2:

I don't think skeletons can see through anything. Only spiders can. But you can toughen up your mob grinder with slabs, that may stop skeletons from shooting at you but at the creepers and other mobs.

Solution 3:

Simply don't stand there. Make a Iron golem the other side. Skeleton would try to hit it. but it will rarely hit it. Iron golems are cheap to make. 4 iron blocks and a pumpkin.