For the quest Railgun Shot (Kill 7 zombies in 1 shot with bullets.) I found it easiest to do it in Don't Touch My Gems mode with the Browning machine gun.
You can use the Browning to move the gems on top of each other (the small explosion from the gun moves the gems a bit. If you want to move a gem down shoot slightly above it, if you want to move it right shoot slight left of it.). Then, wait for the zombies to pile up on the gems and then hit them with the Browning (or shotgun).
I like this game, it's entertaining. I have one thing to admire, the achievements! The large list of achievements is the reason of why we play the game all the day, for complete them, haha that's my case! Well, the quests are good designed. The game has only one mistake, the "level modes", i think that is a little bored playing only 4 levels for so much achievements and quests, i don't know, i would enjoy at least 8 levels. So, the gamers will enjoy all the game without getting bored, that's my point of view
Well, tips:
a) For "ANNIHILATOR": Try to use the browning machine gun, that's a good weapon to unlock the achievements of headshot, 1500 machine guns kills, and much more.. b) For "BULLSEYE": go to the SELECTIONER level mode, and shoot a dart in the center, in the red point. To unlock the another things, use the NUKE with GRENADE, with ANVIL and with LIGTHNING!
Railgun shot: using shotgun, make lots of dead bodies, then wait for zombies to come then open fire.
Overkill: Nuke+Bomb
Just Do it!:
Suicide: Nuke+Lightning Armageddon:Nuke+bomb Bullseye: in level with dart table, hit the middle with dart Fly Anvil Fly!: Nuke + Anvil Zombieball: Ball Rampage, use weak zombies
The hard part of the quests was trying to figure out what they meant. (Is it really inherently understandable that the Suicide achievement means to use lightning on a nuke? Don't think so.) The only achievement that seemed a little tricky (once you knew what to do) was the killing of seven zombies in one shot.
Which is exactly what the Quests should always be like in my opinion! Make sure people actually play the entire game instead of quitting after the last Quest is achieved. This game was a great one and example of what Quests should be like! Hope they will come up with a new one soon!