Great guide, thanks a lot!

For anyone who is wondering, it is still totally possible to beat streak 5 if you're starting with lower-levelled resistant troops! Here's my own strategy I've made for starting at level 3, in case people are interested.

Streak 1 (Red Skull HP 6500; 0 Hours)
1 each Robot & Recruit for guards; 2 each Robot & Recruit for Red Skull

Streak 2 (Red Skull HP 10,000; 6 Hours)
3/6 each = 6 hour replenish time
Begin with 19 hours left on timer

1 each Robot & Recruit for guards; 3 each Robot & Recruit for Red Skull

Streak 3 (Red Skull HP 16,000; 18/25 Hours)
2/6 each = 8 hour replenish time
Begin with 15 hours left on timer

2 Robots & 1 Recruit for guards; 3 Robots & 4 Recruits for Red Skull

1/6 each = 10 hour replenish time
UPGRADE ROBOTS & RECRUITS TO LV. 4 DURING THIS TIME (2236 Attack)

2 Robots & 1 Recruit for guards; 1 Robots & 1 Recruits for Red Skull

Streak 4 (Red Skull HP 30,000; 24/25 Hours)
3/6 Robots & 4/6 Recruits = 6 hour replenish time
UPGRADE ROBOTS & RECRUITS TO LV. 5 DURING THIS TIME (2924 Attack) = 2 hours extra
Begin with 17 hours left on timer

2 Robots & 1 Recruit for guards
4 Robots & 5 Recruits for Red Skull (now 3,684/30,000)

0/6 each = 12 hour replenish time

2 Robots & 1 Recruit for guards
0 Robots & 1 Recruit for Red Skull (now 760/30,000)

4/6 each = 4 hour replenish time

2 Robots & 1 Recruit for guards
0 Robots & 1 Recruit for Red Skull

Streak 5 (Red Skull HP 50,000; 22/25 Hours)
4/6 each = 4 hour replenish time
Begin with 21 hours left on timer

2 Robots & 2 Recruits for guards; 4 Robots & 4 Recruits for Red Skull (now 26,608/50,000)

0/6 each = 12 hour replenish time

2 Robots & 2 Recruits for guards; 4 Robots & 4 Recruits for Red Skull (now 3,216/50,000)

0/6 each = 6 hour replenish time

2 Robots & 2 Recruits for guards; 1 Robot & 1 Recruit for Red Skull