Void Milking
Medium to Longer Challenge - 1 ~ 6 Hour Challenge, depending on efficiency & Upgrades
Strategy
Don't collect any voids - it is not necessary to start with Voidfiend.
Use 
Void Lure & 
Void Radiance throughout the run.
After you've gotten all the Pet Experience upgrades, equip the Voidfiend and collect ONLY 9 Void Entities.
Push through while using the Void spells to get as much Void Mana as you can.
A little trick: This challenge is good for unlocking certain Tier 2 pets due to the reduction in level requirements.
Recommended pet unlocks: Ent, Simulacrum, Arcanaworg.
Strategy: Start out with the Pixie and set up your attributes to at least cover the requirements for Ent, Simulacrum and Arcanaworg. When that's done, spread the rest of your attributes out among Wisdom and Intelligence, then throw excess points into Patience, as usual.
Get your clicker-summons up to autocast and let them run and keep buying sources. DO NOT CLICK VOID ENTITIES! Once you unlock your Ent, let it run for a while. It gives you a couple of quick achievements and income enough to buy the last sources needed to unlock Simulacrum (12,000 sources).
As soon as possible, unlock the Simulacrum and let it run for a while, again, earning you some quick achievements. Once you've spent long enough doing this (at your own discretion), switch to Spellhound and set up spells to obtain and spend as many shards as possible. Conjure Manabeast is your best friend in this run!
Once you have gathered enough spell shards, unlock the Arcanaworg and let it run for a while, again, obtaining you some quick achievements. Enjoy your new pets!
Achievement trick: There's a secret achievement that requires you to cast all Apprentice spells once in order of unlock. Since this challenge decreases level requirements by 60, this is a good place to do that. Once you get to work on the Arcanaworg, you're sucking up shards at a nice rate. Use these to charge all your spells. Remember to not cast any of them.
Once they're all charged, enter them into your spell bar and get them done. Remember to NOT cast Void Automaton until Void Lure has run out! If you catch more than 9 entities, then the challenge is lost and you have to start over.