{duskmantle guildmage} {mind crank}
Activate the guildmage's first ability, then find a way to deal damage or mil a card. One ability resolving will trigger the other for endless pain on turn 4
You can also use {Bloodchief Ascension} in place of {Duskmantle Guildmage} to add consistency.
That actually would work very well. I had been using unblockable creatures like {dimir infiltrator} {invisible stalker} to start the chain reaction
How I picture this combo working
T1 swamp {blood chief ascension}
T2 swamp {Rakdos Shred-Freak} 1 counter
T3 island {dimir guildmage} attack again 2 counters
T4 {mindcrank} attack again 3 counters
Begin combo!
Edit: guild Mage is unnecessary here
I have an entire deck built around this combo and I can tell you it works perfectly. So much so I am the first target it multiplayers
The guildmage is perfect for the combo with mind crank especially since blood chief takes a while to charge. But if you need it to charge (or you need something to do damage to initiate the infinite loop) I suggest stab wounds. They deal 2 damage to the player every turn and cripple a creature slightly.
The player is forced to try and kill off the stab wound or play their counter spells etc.
Unblockable is also strong with trepanation blades. Still can't hit them to force the combo? Use codex shredder to mill for one card and so on. Codex shredder can also be used to retrieve a blood chief from the graveyard just in case the enemy naturalises it!