Let Them Come (UP)

This achievement is for killing the Smasher, which is the first boss on wave 25.

I thought this guy was initially impossible and I tried a number of loadouts. Here is what worked for me....

Passives
* Red Cell Shot - More health
* Body Armor - Take less damage from projectiles
* Rapid Fire - Fire faster, but downside is more heat and more spread - remove if these bother you.
* Cooling - Gun creates less heat

Secondary
* Knife - I didn't use this on the boss

Ammo
* Standard Ammo
* Piercing Ammo with Charge Mod MUST HAVE

Ok... So, the actual ammo isn't as important. You can use shock, explosive or cluster, but the key is the charge mod. Charge mod costs 400 and is on the left column. When you buy it, hold to add to loadout and choose the piercing ammo (or other good ammo). I found piercing ammo the best because it's cheapest.

Honestly, the boss will die fast if you spam charge shots at him (I aimed for head). The problem with this is that if you fail, it's expensive to replace all the ammo and that's what annoyed me at first.

The boss has a few different attacks....

* Ground Pound - This is easy to identify as a big ! appears above his head. If you shoot him, it will interrupt his ground pound (aim for the head - body is inconsistent).
* Lob single purple grenade - This annoyed me because it's inconsistent to shoot and the riot shield addon doesn't block it. Pure and simple... aim as high as you can and hope you hit it. I don't think there's anything else you can do here.
* Egg Spawner - The boss will crouch and fire out some purple eggs that will stick to the ceiling and floor. You need to shoot them before they hatch and spiders come after you (this is also the only decent way to farm money on this stage). You can use grenades on these, but I just shot at them as much as I could. One thing to note though, while the boss is crouching down, his head is very vulnerable, so you can do the most damage by hitting him here.

Good luck.

Anyone new to this game, a lot of the stuff besides ammo and grenade's can be upgraded 3 times. I didn't know that when fighting the boss at the end of level 1 and he kicked my butt many times. This game doesn't really seem to have a start over at beginning option. So upgrade the gun, the gun cooling and your health a few times before buying any grenade's or melee items, that is my opinion.
You can always grind a level you are stuck on for gold but it might not pay off very well. I couldn't beat the level 25 boss until I started tossing the $1000 best grenade's, black hole. And I bought the item that lets you throw grenade's and melee faster. I wasted a lot on other grenade's and bullshit melee items not knowing that I could upgrade the big gun past level 1.