Free character should not have an easy time killing tanks, then there would be no point in spending money on tanks when free people an kill them. Keep in mind, while free characters have a hard time with heavy armor, they can still toss timed C4, and (for example) free flamethrowers can make short work of light armored vehicle, like MRLS.
As for heavy armor, once you get past free characters, there are a number of purchasable character that work awesome. Gunners, Laser Chaingun/Rifle, Railgun/PIC, then the volt rifles...soon...