Unstoppable Force is one of the best initiating abilities in the game, as it transports Malphite to the target location and knocks up all nearby enemies. Malphite is also immune to disables during the dash.
Remember to allow Granite Shield to recharge if a team fight is about to start.
Malphite excels at being an anti-carry, designed to stop carries in their tracks with his Ground Slam and then destroy them with his high damage output.
Malphite can make a strong counter to certain solo top champions, such as Tryndamere, Irelia, or Jax that have few ranged options and need to auto-attack to secure lane control. Use Seismic Shard to harass and Ground Slam to interfere with last hits, giving you free reign to farm yourself.
If your enemy is on low health and they are behind a wall, Unstoppable Force can go through walls and you can pick up a kill easily.
When jungling on Malphite, it is crucial to reach level 6 quickly. A gank with Unstoppable Force is extremely devastating and can turn even slippery laners such as Fizz into easy kills when used correctly.
It should be noted, however, that his passive, Granite Shield, scales with maximum health, so planning on building a health item first, even if you are unsure what your endgame build will be, is a wise choice. This will also grant you some early game tankiness and survivability to help you through laning phase.
Malphite is typically built for a tank role but can be built with ability power, trading off late game utility for damage.
Deathfire Grasp is a strong choice if it is your goal to assassinate single high priority targets. The active can be used during Unstoppable Force to massively increase the burst damage of Malphite's combo.
Note that the health regeneration is pathetically small during the actual teamfight. It will get you at most +120 health for a fifteen second teamfight. Health Regeneration is much better during the laning phase, and drops off significantly in usefulness during teamfight situations.
You might even be able to zone Malphite a little while he waits for the shield to refresh.
Don't even try to go against Malphite as a melee autoattacker because that is exactly what he excels against with his Brutal Strikes armor and attack damage steroid and Ground Slam attack speed reduction. These alone make him heavily favored in a trade, not to mention his Granite Shield.
Malphite's strengths can be effectively countered by going double AP, as opposed to the usual bruiser meta.
Note whether or not Malphite is spamming his abilities (especially Seismic Shard) as he isn't exactly the most mana efficient champion around.
When chasing or being chased, it may be a good idea to hold on to any movement speed increasing buffs you may have until after he has used Seismic Shard, as increasing your speed will also increase the amount of movement speed he steals and essentially nullify your boost.