For figuring out how much cash you need to get a certain number of Angel Investors total, use his formula:
((Desired Investors)^2) * (10^13) / 225 = Total All Time Cash Needed
You'll get your first investor at 44,444,444,445 (44.4 billion) Total All Time Cash, your hundredth at 444,444,444,444,445 (444.4 trillion), and so on. You only actually get the use of them when you reset the game (NOT Hard Reset!), and it tells you how many more you'll get if you reset.
For the "Deal 250000 damage"challenge, overkill damage is counted as well. What I did was place a bunch of guard towers, and had them all be the shock/chain (whatever its called) tower, With +30 waves and double enemies, they did enough overkill damage to get me like 270k total damage.
Fun, and I really liked the music. Didn't have to go back to any locations, just kept hitting retry until I managed to beat it without dying. Difficulty of the final boss was kind of sporadic, some forms were really easy and some were much harder than others.
 "AdVenture Capitalist"){kind=icon}
 "Hands of War Tower Defense"){kind=icon}
 "Bullet Audyssey"){kind=icon}
 "Bloons Tower Defense 4"){kind=icon}
 "Level Up!"){kind=icon}
 "The Space Game"){kind=icon}
 "Penguinz"){kind=icon}
 "Bowmaster Prelude"){kind=icon}