Parking lots are fine until you come against something that can really wreck your day. As a guard player I tend to actually use my chimeras to capture objectives, so i don't tend to park them all side by side like this.
A multi charge with a unit of terminators should destroy a big bunch of tanks all lined up next to each other. They can deep strike too. This seems pretty straight forwards as a counter strategy to me.
Sure if all you use are cookie-cutter internet "rifleman festival" style armies, autocannons are going to ping off armour 12 for the most part. That serves you right for not bringing a mix of weapons to the table. There is a reason lascannons are expensive, and it isn't just to encourage you to only field autocannons. [despite what the internet will have you believe]
Marines can field razorbacks with several anti tank weapons, which are far more capable of busting chimeras, than chimeras are of busting them back. Marines also get Multimeltas on a bunch of fast moving stuff, eg attack bikes, landspeeders, and can attach them as a spare machine spirited gun on all their landraiders.