BTW; I just remembered I started compiling a list of what's in each challenge, since I couldn't find such a list anywhere else. It's not exactly complete (most of the non-standard challenges only list the type of challlenge, not content or differences between them), I should get around to completing it sometime.
01: Cars + Planes (3 waves)
02: Constructicons (all six), Devastator
03: Insecticons
04: Cars (3 waves), Tanks
05: Cars + Planes + Tanks + Insecticons (2 Waves)
06: Planes (4 waves)
07: Cars + Planes + Tanks (3 Waves)
08: Cars + Planes + Insecticons (2 Waves)
09: Starscream + Blitzwing
10: Starscream + Skywarp + Thundercracker
11: Motormaster + Cars + Tanks, Menasor
12: Soundwave + Shockwave
13: Megatron + Starscream
14: Bumblebee, Sideswipe, Wheeljack, Grimlock, Optimus
15: Megatron + Shockwave
16: Megatron + Optimus
17: Bumblebee + Grimlock, Sideswipe + Wheeljack, Optimus
18: Blitzwing + Soundwave, Starscream + Shockwave
19: Cars + Jets + Tanks (many at once), Devastator
20: Motormaster, Skywarp + Thundercracker, Blitzwing, Giant Skrapnell, Starscream, Shockwave, Menasor (with waves of normal enemies in between)
21: Bumblebee, Giant Bombshell, Grimlock, Giant Skrapnell, Sideswipe, Giant Kickback, Wheeljack, Optimus, Hook + Bonecrusher + Scavenger, Long Haul + Scrapper + Mixmaster, Giant Insecticons (all three), Motormaster, Shockwave + Blitzwing, Bumblebee + Wheeljack, Soundwave + Starscream, Grimlock + Sideswipe, Megatron, Optimus (with waves of normal enemies in between)
22: Head to last level
23: Overhead view, Cars + Planes + Tanks
24: Overhead view
25: Overhead view
26: Overhead view
27: Overhead view
28: Protect your allies
29: Protect your allies
30: Protect your allies
31: Protect your allies
32: Protect your allies
33: Reinforcement Battle, Starscream + Skywarp + Thundercracker
34: Reinforcement Battle
35: Finish in time
36: Finish in time
37: Finish in time
38: Finish in time
39: Finish in time
40: Punt the Decepticons
41: Punt the Decepticons
42: Punt the Decepticons
43: Punt the Decepticons
44: Defeat as many as possible
45: Defeat as many as possible
46: Defeat as many as possible
47: Finish in time
48: Finish in time
49: Defeat the Insecticons (Galaga)
50: Defeat the Insecticons (Galaga)