You might want to consider to give the players more information like how much more immunity you get from a shield, the speed of Robert, how much faster another bow makes you shoot and so on.
You could also give the players a choice at the start of each game, to get a choice between 4 different "special abilities". Like single-target arrows (high attack-speed), arrows that spreed (the more bows you get the wider the spreed shoot gets) or actives/toggle-on abilities like "time freeze", slow arrows, fire (burn) arrows or increase attack-speed.
Suggestions
All the different items should have an icon and a x and then the amount that you have of the item, instead of just piling up next to each other.
You should also make it so that you can't buy health at full hp, coins should be a bit more rare or you make a shop where you can buy better things (upgrade stuff) after the game ends (you could also make it so that you would receive a gold reward if you clear the game, higher reward for higher difficulty).
Great game, but it needs some work. First of make a bar that shows the boss health. 2nd fix the backward crossbow.
Bugs
I encountered some bugs in the game. The first one was "time slow" at some stage the game slowed down about 33% (might have been that the fps were reduced, not sure). The bug fixed itself after the next boss level.
The second bug that i encountered was that the ghost boss got stuck and couldn't move ( I could still deal damage to it, but it was really weird)
I just experienced a bug. I've got a bug on firefox which makes a lot of webplayers crash instant. The game adds you can watch is one of them. What happened was that I collected my offline rewards (around 5 million skulls). I was then asked if i wanted to double my skulls since it was a lot. You know, the greed kicked in :p and it was a game add that was shown. My game crashed and the 5 million skulls were lost :/.
Bob's texts on his upgrades are wrong. In the first upgrade it says "For the Horde!: Add 25% to Bob The Necromancer DPS", when it should say "For the Horde!: Add 25% to Bob The Necromancer DMG" since Bob doesn't have DPS but damage pr. click. It's the same with upgrade 3 and 4. Change DPS to DMG :)
Great game. I hope you do something to the hobbies soon. Besides that, you might want to consider removing or redoing some of the achievements:
"Put a Ring on it" (play the game for a year)
The problem for the time played achievements is that it isn't "time played" but time spend online, which is insane.
(It is not like I'm going to leave my computer on and the game running for a year, right?)
"OG" (Give at total of 100.000 gifts)
Since you can swap between the girls when giving gifts, it still takes about 3 seconds to give a gift, swap and give another one.
If it takes 3 seconds to give a gift this achievement would take 83.33 hours to do. (About 3.5 days)
"Ace of Hearts" (Tap for a total of 1.000.000.000 hearts)
First, impossible.
Second, impossible.
Third, rip mouse.
You might also want to change this achievement since the only way to achieve this with out ruining your arm, fingers or mouse, is by using a bot (and we don't want that to happen)
Best of luck with the game
 "Robert The Elf"){kind=icon}
 "The Perfect Tower"){kind=icon}
 "Zombidle"){kind=icon}
 "Crush Crush"){kind=icon}
 "Midas' Gold Plus"){kind=icon}
 "Midas' Gold"){kind=icon}
 "Tap Heroes"){kind=icon}
It is really weird, we don't know yet what he does with them but he seems to love the blue stones :P