That's quite a difficult situation to win in :/ If you've allowed him to reach this point in your game and he just makes a giant ball and slowly advance towards you there are only a few things you can do to try and break him.
1, I'm assuming you both kinda just turtled to get into this b/c with constant back n forth amounting a force that large is pretty rare. That being said you need to have some good tech to back you up and not just a million hydras and lings which will promptly be blown sky high. Zerg in general to me is all about divide and conquer, proper flanking, and knowing how to do hit and runs.
So lets focus on what you need to kill and counter to win the engagement.
Mutas > extremely agile and fast, fly around to his rear and take pot shots at his tanks, ONLY until he either turns his mmm ball or thors around towards you, you never want to fight those with only mutas as you will be absolutely decimated.
Infestors > ESSENTIAL spell caster, fungal growth, neural parasite aaand new frenzy spell (25% more damage and immune to snares 30 seconds; eg. marauder concussive shot/thor barrage thing) You'll want a handful of these guys to primarily fungal growth his mmm ball with not only slowy damages them overtime but immobilizes them from running around, plus they're already probably packed in super tight around eachother making fungal growth all that more effective. Parasite you have to use on the guys that count, so thors and tanks, though be sure to have a stream of other guys in there to absorb damage (more on this in a bit) while your caster runs to the middle of the fight. DON'T forget that you have access to abilities of a parasite unit, so you can use the thor barrage attack if the terran researched it on his own tanks
Roach(in particular),
hydra,
bling > The roach is the only unit that's armored in these three that in a normal game would be used to absorb 2 or 3 tank shots before going down while your real damage dealers, zerglings/hydras take out the enemy. Of course since he's amassed such a large force this gets more complicated. You simply need a shitton of ground units to overrun him, try and get roaches in first to take the blunt of the blows. This again is where flanking comes into play as well. In fact if your ballsy enough and know he doesn't have say a raven, detector, with him you can even try to move burrowed roaches right under him and come up right as the action begins on his mmm force, the AI of his marines and marauders kinda goes haywire for a few seconds as they move around, but you have to time it perfectly so that your main force also attacks at the same time. Speaking of which, you can't send in your army single file to the fight. spread your guys around so they make something like a wall when you send them in, even using multiple hotkey groups if you can to control the masse of units a bit better.
I did mention bling (zergling/baneling) which you can run right in, a second or two after the main force moves so they won't all die before they get there to kamikaze some marines, keep in mind banelings have a pretty decent area explosion when they pop on his marines so you don't need an excessive amount. Also remember banelings are shit for daamge to a marauder, so AIM for the marines
Bonus >
Ultras > Now if you have an army composition of what I described above you've already invested quite a lot of money already so you may not have enough for this additional tech. With this new patch they should theoretically be more versatile, frenzy them and hit the main line of thors where their new damage of 15+25 vs armored will reall come into play, if the thors are disposed of and you still have ultra, move them to tanks next, remember to attack what you deal bonus damage to to make the most of your army.
*whew* Now of course writing all that and you executing it is all very different. It takes quite a bit of coordination to pull off an attack like I described, you have to be very very good at using hotkeys and fast! Particularly the infestors, F for fungal growth, E for parasite. Cross your fingers and hope for the best, gl
PS TLDR, yeah whatever
