Discard a Level 5 or higher DARK monster to Special Summon "Dark Grepher", then activate its effect, discarding a DARK monster to send another from your Deck to your Graveyard, thus setting the 3-monster requirement to Summon this card.
Use the effect of "Evil Thorn" to Special Summon 2 more copies of itself. When both of those monsters have been sent to the Graveyard as well, there will be the 3 required DARK monsters to Summon this card.
Attack more powerful monsters with your "Giant Germ" or "Mystic Tomato" to get copies of themselves to fulfill the number of DARK monsters required.
Use "A Wingbeat of Giant Dragon" to return this card to your hand and destroy all Spell and Trap Cards, then Special Summon this card again and use its effects to destroy the opponent's monsters.
Use the effect of "Advanced Dark" to send "Crystal Beast" monsters to your Graveyard, which will become DARK to fuel this card's Summon condition and effect.
If there is a "Mystic Tomato" or "Giant Germ" on the field, attack your opponent's monster with them to achieve the requirement for Summoning of "Dark Armed Dragon" easily at the cost of some Life Points and your Battle Phase.
Discard 3 DARK monsters to Special Summon "Montage Dragon" and Summon this card. If high-Level cards are discard such that "Montage Dragon" has at least 5200 ATK, this can create an OTK after this card clears your opponent's field.
"Kycoo" might need some support to prevent it from being destroyed by battle, though.
"The End of Anubis" and "Necrovalley" prevent Graveyard manipulation, especially "Necrovalley", which prevents this card from using its effect as well.
"Dimensional Fissure" and "Macro Cosmos" prevents monsters from entering the Graveyard, preventing both this card's Summoning (if the opponent doesn't have 3 DARK monsters) or further destruction if this card is already in play.
This card works well with "Cold Wave". It can destroy your opponent's Spell and Trap Cards without risking their activation.
In a "Disaster Dragon" Deck, playing "Future Fusion" while "Dark Armed Dragon" is in your hand can easily set up a possible OTK. "Future Fusion" can send five Dragon-Type monsters to the Graveyard. Two of them would be "Red-Eyes Darkness Metal Dragons", another two would be "Red-Eyes Wyverns", and the final one would be another DARK Dragon-Type monsters such as "Dark Horus". After activation and all five Dragon-Type monsters are in your Graveyard, Special Summon "Dark Armed Dragon", then set some cards face-down for protection, and then end your turn and banish two "Red-Eyes Wyverns", which will Special Summon both "Red-Eyes Darkness Metal Dragons". On your next turn, Special Summon even more Dragon-Type monsters.
If your Graveyard has too many DARK monsters use the following:
"Tour Guide From the Underworld" can Summon "Sangan" and when either player activates "Torrential Tribute" or "Dark Hole", the effect of "Sangan" will add "D.D. Crow" from your Deck to your hand. Use the effect of "D.D. Crow", then banish any non-DARK monster or Spell and Traps in your Graveyard or any card in your opponent's Graveyard to Special Summon "Dark Armed Dragon".