If you're playing against space marine players, it's a pretty safe bet they're running either drop pod grav or battle company. The easiest counter to that is Haemonculus Covens units, specifically Grotesques and Wracks. If you're still invested in 7th edition and want some new models to try out, I recommend the Scalpel Squadron formation. It consists of 2 Venoms and 2 units of Wracks. The new plastic Wrack kits let you build Venom crewmen out of the spare parts too so if you're looking for consistency with your dudes there you go. Otherwise, Grotesques tend to work very well against Space Marine players, particularly if they're using a lot of grav or Rhinos.
Really terrible videos about tiny plastic space elfs intended to help you get gud scrub:
Flawless pieces of literary perfection: