Armaldo is an interesting case. It has all of the makings to be a pretty good UU tier Pokemon, but it still falls into oblivion in comparison to Rhydon, who boasts a superior type combination, a resistance to Stealth Rock, higher attack and defense, and can hold the Eviolite.
So why use Armaldo?
Rhydon may have a better type combo, but not on every occasion. That bug-type can allow Armaldo to hit the now popular psychic type with STAB X-Scissor. More importantly, Armaldo has the ability Swift Swim, making it a speedier non-water rain sweeper. It also has Aqua Tail, giving it a third psuedo-STAB move to use. The final advantage over its older cousin is access to Rapid Spin and higher special bulk without Eviolite factored in.
Play Armaldo to its strengths and it won't disappoint. Just don't expect too much out of it.
Outclassed by Terrakion
Ability: Swift Swim (always)
EVs: 252 attack and speed, 4 hp
Nature: Jolly
@: Life Orb/Leftovers
Swords Dance
Rock Polish
Stone Edge/Rock Blast
X-Scissor/Aqua Tail
Double stat boost sets are tricky, but with Swift Swim, Armaldo can pull it off with its bulk backing it up. Boost and go. Stone Edge hits hard, even without Swords Dance, but Rock Blast has better accuracy and can break subs. X-Scissor is secondary STAB, but Aqua Tail hits very hard in the rain and will do more neutral damage than X-Scissor in the rain. Not only that, Aqua Tail smashes ground types that resist Stone Edge/Rock Blast.
Band? Scarf? You decide.
Nature: Jolly/Adamant
EVs: same as last
@: Choice Band/Choice Scarf
Stone Edge/Rock Blast
X-Scissor
Aqua Tail
Earthquake/Superpower/Low Kick/Rapid Spin
Slapping a choice item on Armaldo became much more doable with Swift Swim in its arsenal. Choice Band loves Rock Blast, while Scarf is better with Stone Edge. Earthquake lets Armaldo hit Metagross, which can Meteor Mash Armaldo with relative ease. Superpower and Low Kick hit Tyranitar harder than Aqua Tail while giving coverage on Ferrothorn, who walls this set otherwise. Rapid Spin does what Rapid Spin does.
A more Passive approach
Nature: Careful
EVs: I need help for this one.
@: Leftovers
Stealth Rock
Rapid Spin
Rock Blast
Toxic
Not a standard support set, but it can work. With Swift Swim, Stealth Rock is sure to hit the field. Rapid Spin is Rapid Spin. Rock Blast is the offensive move because of the higher PP, sub breaking, and higher accuracy. Toxic spreads status and is the best way to shut down heavily defensive stuff. Shame Armaldo doesn't get Dragon Tail.
Other stuff:
Substitute is an all-round useful tool on practically every occasion.
Battle Armor prevents crits, which helps bulky variants. However, crits don't happen often enough to sacrifice the potential speed boost from Swift Swim. On sandstorm teams, Battle Armor is the way to go, but that's when Rhydon becomes a much better choice.
Rock Slide can be used over Stone Edge or Rock Blast for accuracy and flinching and, in the case of Rock Blast, more consistent damage. Not a good trade-off though.
Using just Swords Dance is an option, but Rhydon does better.
Rock Polish looks better than Swords Dance thanks to Swift Swim, but Armaldo won't be hitting very hard.
That's Armaldo.