To be fair, there have been a lot of nerfing of Venam lately especially on easy.
-Geodude available in the sewers
-Removed Merciless on Mareanie [easy]
-- Changed first badge cap to 18- Adjusted Venam's levels slightly to match new cap; removed Venoshock on Mareanie
What I recommend is pickling up the voltorb [buy 4 poke balls from the market dude that asks for $400/ball], and teach it Charge Beam when it levels up enough, then lower Mareanie's Sp Attack with Eerie Impulse. Enough charge beam boosts tears through her team