Hint: Faster Reload 
     Instead  of  reloading normally, go to the next weapon, then back
to the previous weapon. This reloads weapons that reload very slowly.

Unlock Characters In Story 
     Beat the level and unlock these secret playable characters:

Medium level difficulty
2280 Return to Planet X: Ozor Mox
2019 NeoTokyo: Sadako
1853 Wild West: The Colonel
1972 Atom Smasher: Khallos
1920 Aztec Ruins: Stone Golem
1990 Siberia: Hybrid Mutant
1938 Chicago: Big Tony
???? Notre Dame: Jacque de la Morte
2280 Return to Planet X: Ozor Mox
2019 NeoTokyo: Sadako
???? Robot Factory: Machinist
???? TimePortal Station(?)