If it's a regular group and they think your army is too strong, switch armies with them for a game or two. Let them pick from your Liz collection and make a list out of their collection. Show them how powerful their own stuff is and how to do it. I played a campaign with my lizardmen where everyone kept saying they were over powered, even as I was telling them exactly how to beat my army. It is frustrating.
To be honest, next time somebody tells me that I'm OP, I'll just design them an entire army for their faction that's just as strong as mine on the spot to prove them wrong, then tell them to use it.