I can give you one tip to lvl fast.
Naga helped us a lot by giving to certain mobs a 100% chance to drop very special mods on death:
- Rat-monkey:Relics (Holborn access shafts from Holborn station).
- Sleestak: Fuels (Upper Kingsway sewers from Covent Garden Station).
- Mini Dynamo: Techs (Green Park approach from Charing Cross Station or Green Park Station).
These mods are very powerfull and can Boost your clear speed significantly.
Rat monkey relic is particulary godlike. It's a "divine" relic ( 12/14% increased Damage) with random augs.
By the way, if you or someone else have any clues on where I can find the 3 other types (Ammos, rockets, batteries) , please let me know.
Hope it helped.