Malphite & Senna Synergy
Malphite Top and Senna Support win 53.7% of 3,646 Low Elo games together, +1.4 points vs expected, 2-patch window 26.19-26.20.
Researched on LoL patch 26.20. Tips are written to hold at any rank; stats come from Low Elo ranked games up to patch 26.20.
Why this duo works
Senna's Curse of the Black Mist(E) can conceal Malphite's approach and give him a surprise angle for Unstoppable Force(R); Senna can follow the engage with Dawning Shadow(R) to shield him and damage enemies. They should approach from an angle that does not telegraph the whole team grouped in the mist.
Playing the duo
Curse of the Black Mist(E) camouflages nearby allies and grants movement speed, giving Malphite a hidden approach for R; Dawning Shadow(R) can then shield the dive and hit enemies in its beam.
How to play as Malphite
Use Senna's mist for a flank or delayed approach rather than walking directly into a visible group; her R can support your engage once you go in.
How to play as Senna
Use E to cover Malphite's approach, then follow his R with Dawning Shadow(R) to shield him and damage the enemies caught in the engage.
Sources
These notes come from r/leagueoflegends, checked against the champions' abilities.