If I'm reading the wiki correctly and my math is right, then suggesting that the people who cant do Reunion G should farm Artillery Monsters X over Army of the Dead X for souls is poor advice. You're less than likely (~46%) to hit all 9 souls per run in Artillery. You're correct that the 5 soul drop in Army is only 30% as opposed to 90%, but they're two of them while also guaranteeing 6 souls at 100% per run for a potential maximum of 16 in a single run. Assuming you want the most possible souls without having a team that can farm Reunion G, you should be putting all of your stamina into Army of the Dead X. If you can do Reunion G then its the best option for soul farming assuming you don't need silvers. This information is all available on the wiki as well.

TLDR: Read the wiki before helping out. Army > Artillery