
Reviewed for patch 7.3b

Favorable matchup
Reviewed for patch 7.3b
Fiora hits both pillars of Malphite's kit: Vitals deal true damage based on his max health, so Thunderclap's armor does nothing against them, and Riposte can block Unstoppable Force and send a stun back.
Thunderclap's passive gives Malphite armor, and his damage grows with it. Striking a Duelist's Dance Vital deals true damage based on his max health: the more health he buys, the more every Vital you land with Lunge is worth.
Riposte parries all damage and effects for a moment and stuns if it blocks an immobilizing effect. Time it for when Unstoppable Force reaches you and you block his damage and the knock-up and stun him in return. Use it early and Malphite can simply wait and charge once it's gone.
Ground Slam lowers the attack speed of anyone it hits for a few seconds. Against Fiora that cuts into Bladework. Strike the Vital with Lunge and step out of the area instead of trading slowed basic attacks.
Go in while Unstoppable Force is on cooldown, or while you hold Riposte for his charge. After he uses Ground Slam, back off until the attack speed reduction wears off, then come back for the next Vital.
If you're on Malphite, don't charge into a Fiora who still has Riposte: fake it, wait for her to use it, or save Unstoppable Force for a fight where another ally forces it out. In lane, use Seismic Shard to steal her speed when she goes for a Vital and Ground Slam when she stays to trade. Granite Shield needs time without taking hits, so step back to get it back before the next trade.

Favorable matchup
Reviewed for patch 7.3b
Mordekaiser deals almost all of his damage as magic damage, which Malphite's armor doesn't reduce, and Realm of Death pulls him out of the teamfight before his charge can reach several allies. In lane, his shield soaks up part of the Seismic Shard poke.
Obliterate and the Darkness Rise aura deal magic damage. Thunderclap's passive and the armor items Malphite usually buys don't reduce it: he has to buy dedicated magic resist or accept trades that are worse than his health bar suggests.
Realm of Death drags a champion into another dimension with Mordekaiser and steals part of their stats. If you catch Malphite before he uses Unstoppable Force, his charge can only hit you inside the realm, not your allies in the main fight.
Malphite wins lane with Seismic Shard from range. Indestructible turns the damage you deal and take into a shield, and you can convert part of it into health: use it after eating several shards, not at the start of the wave.
Trade when Malphite walks into Death's Grasp range or uses Ground Slam on the wave. Before an objective fight, save Realm of Death for him if he still has his ultimate.
Malphite needs magic resist and patience here: poke with Seismic Shard from outside Death's Grasp range and don't accept long trades inside his aura. Granite Shield recharges while you take no damage, so step back between waves. In teamfights, charge once Mordekaiser has used Realm of Death, or from an angle he can't reach before you.

Execution matters
Reviewed for patch 7.3b
Kennen hits from range with magic damage and keeps Malphite without Granite Shield. It's a conditional answer: it depends on not eating Unstoppable Force with no way out, because Kennen is fragile and Malphite can engage on him with his jungler.
Malphite's shield only recharges if he goes a few seconds without being hit. Thundering Shuriken and Kennen's ranged basic attacks keep it broken, so every trade starts against his health, not his shield.
Kennen's abilities apply Mark of the Storm and stun at three marks. All of that damage is magic, so Thunderclap's armor doesn't reduce it. Ground Slam lowers your attack speed, but your main damage comes from abilities.
Poke when Malphite walks up to last-hit and you have Lightning Rush available. Once he has his ultimate, check where his jungler is before every step forward.
On Malphite, your best tool is the coordinated ultimate: Kennen can't survive a charge followed by your jungler's damage. Use Seismic Shard to steal his speed once he spends Lightning Rush, and don't chase through the wave while eating shurikens. Early magic resist on boots or an item takes a lot out of his poke.