Armaldo is an intersting Pokémon. Its typing is rare, only shared by Shuckle and the Crustle line, and has several Water moves and Swift Swim, allowing it to somewhat function well in the rain (being weak to water moves rarely ends well in the rain). However, its weaknesses are common, and a tanky pokémon without any reliable recovery that is weak to Stealth Rock is going to struggle.
Ancient Guardian
Armaldo
Ability: Battle Armor
Item: Leftovers
-Swords Dance/Curse/X-Scissor/Rock Slide
-X-Scissor/Rock Slide
-Rest
-Sleep Talk
EVs: 252 HP/252 DEF/4 ATK
Nature: Impish (+DEF, -SATK)
Physical Wall Armaldo. Come in on a Physical threat, sponge the hits and KO them with your own high Base ATK. Battle Armor ensures you can't be crit. Swords Dance and Curse make up for the fact that you didn't invest much into attack. X-Scissor and Rock Slide are for STAB (Rock Slide is for the more PP over Stone Edge). RestTalk Rounds out the set, giving Armaldo recovery and a way to bypass unlucky burns. If you are choosing a mono attack set (RestTalk + Setup + attack), X-Scissor is more reliable while Rock Slide has much better coverage..
Other Options
Aqua Jet -Priority Option to patch up Armaldo's shoddy speed
Hone Claws (Transfer Only) -Why do rock moves have less then 100 accuracy? Well, this fixes that at least.
Screech -Alternative to Swords Dance, works well with a hazard setter, scaring opponents out and against predicted switch ins, so even if the pokémon sent out completely counters Armaldo, you made it that much more vulnerable to revenge killing.