I mean the Calm Mind/Recover/Refresh/Judgement sets I keep running into, usually run by Dark/Rock Arceus.
There are several ways to beat Mono-attacking Dark Arceus with stall teams
1) Scarf Terrakion. You can fit it in over Arceus or Lugia depending on who you need more. Usually I will recommend making Giratina Specially Defensive and dropping Arceus for Terrakion. Terrakion/Groudon/Lugia already stomps Extremekiller anyway.
2) Scarf Heracross. Same as Terrakion except you can now OHKO Mewtwo/Darkrai easily in exchange for checking SD Arceus with it. Your team is also pretty weak to Mewtwo, Arceus will just get Psytriked, Giratina and Lugia will be Ice Beamed. Although it won't OHKO either walls, you can only phaze it out once.
3) Steel Arceus. You can use Calm Mind Steel Arceus with Will-O-Wisp or Roar to deal with Dark Arceus. Steel Arceus will win in the end due to the typing advantage or if you use Roar, you will easily triumph. Perish Song is fine if you like to use it.
4) Blaziken. Protect on the switch in and OHKO Arceus Dark next turn or just SD right way and kill it. You can probably drop Arceus-Water or Lugia for this. Gives you a Darkrai or Mewtwo(with a Jolly nature) check. Keep in mind your team is quite weak to Manaphy if Groudon is down.
5) Ferrothorn + Chansey/Blissey. Leech Seed annoys the hell out of Arceus if it lacks Sub. If it has Sub, Toxic Spikes can ruin it. Chansey can just simply Seismic Toss Arceus to death with Leech Seed Support and keep on spamming Toxic as Arceus Recovers, forcing it to use Refresh.
6) There are some other checks such as Ho-Oh(SR out means you die), CM Kyogre(not a reliable check if hazards are down), BU Dialga, Skarmory(can't beat Arceus in the end), Heatran. Oh yeah just saying CB Gyarados beats Arceus one-on-one in the rain. SD Excadrill beats Dark Arceus as well. Tyranitar can eventually beat Arceus with Low Kick.