I guess I could write it down step by step.. Here's how I start off atleast:

Start - Build 2x Guntower (Upgrade one) and 1 Grenade tower. (0 Credits left)

Round:

1. Gain 20 credits (20 Credits left)
2. 41 credits - Upgrade other gun tower (11 credits left)
3. 56 credits - Upgrade 1st gun tower again (16 credits left)
4. 63 credits - Upgrade 2nd gun tower again (23 credits left)
5. 72 credits - Upgrade Grenade tower (12 credits left)
6. 88 credits - Build gun tower & upgrade it (28 credits left)
7. 108 credits - Upgrade gun tower again, build grenade tower (8 credits left)
8. 90 credits - Upgrade grenade tower (30 credits left)
9. 183 credits (Boss round)
10. 354 credits - Build gun tower and upgrade it twice (254 credits left)


I'll finish documenting later.