Attempt to draw card, instead choosing to mill 3 cards and return Shambling Shell from your graveyard to your hand.
If you milled a nonland card, The Wise Mothman triggers, putting a +1/+1 counter on Benthic Biomancer.
Benthic Biomancer triggers, causing you to draw a card and discard Shambling Shell.
Repeat.

Infinite self-mill

Benthic Biomancer

The Wise Mothman

Shambling Shell

Attempt to draw a card. 
The draw will be replaced by Chains of Mephistopheles, instead causing you to discard Shambling Shell. Then, the draw will be replaced by Shambling Shell's dredge ability, causing you to mill cards and return Shambling Shell from your graveyard to your hand.
Dredger's Insight's first ability triggers, causing you to gain 1 life.
Lich's Mastery triggers, attempting to draw a card.
Repeat from step 2.

Near-infinite lifegain

Near-infinite lifegain triggers

Near-infinite self-discard triggers

Chains of Mephistopheles

Dredger's Insight

Lich's Mastery

Put a +1/+1 counter on Benthic Biomancer.
Benthic Biomancer triggers, causing you to mill three cards and return Shambling Shell from your graveyard to your hand, and then causing you to discard Shambling Shell.
Thran Vigil triggers, putting a +1/+1 counter on Benthic Biomancer.
Repeat from step 2.

Thran Vigil

Activate Shambling Shell's cycling ability by paying {0} and discarding it, causing you to mill a number of cards and return Shambling Shell from your graveyard to your hand due to its dredge ability.
Repeat.

Rhet-Tomb Mystic

New Perspectives

Activate Kiki-Jiki by tapping it, creating a token copy of Shambling Shell with haste.
Activate the Shambling Shell token by sacrificing it.
When the Shambling Shell token dies, Kiki-Jiki triggers, untapping itself.
Resolve the Shambling Shell ability, putting a +1/+1 counter on target creature.
Repeat.

Combos with "Shambling Shell"