It can be very difficult for parents of Autistic children to deal with, but this is a fact: tantrums are unstoppable. And in the case of children with Autism, the tantrums are SO MUCH MORE INTENSE.
If you are a parent of a child on the spectrum, I am about to tell you something you won't want to hear: the tantrums don't go away with age. I'm a full grown adult (sort of), and I still throw tantrums when I lose my keys.
The good news is that it's not your fault. You may feel like you are causing the tantrums by either using discipline or holding firm to boundaries, but it's not that. Sometimes it can be the smallest thing that sparks a tantrum. For me, it's losing something. For someone else, it can be an uncomfortable setting (too warm/too cold/too loud/too quiet/etc.). Some kids can throw a tantrum over a broken toy. Some can go off because they didn't get the right amount of chocolate chips on their cookie. It's different with every Spectrumite.
So, how to deal with the tantruming child? Well, let's start with what not to do. First, don't tell the child to be quiet or to not be upset - this will only make the child feel alienated and "wrong" for being upset. Second - and trust me on this - do NOT let them "cry it out." Tantruming Spectrumites very often get physical during tantrums and can cause harm to objects and themselves. If you just let them get it all out, you may end up with a broken lamp or a broken body part.
The best course of action is to hold the child tight and let them know everything is going to be okay. This is where wrestling skills come in handy. Your youngster is going to fight back; don't take it personally. But, BE CAREFUL!!! Remember, you are trying to prevent harm, not cause it. So make sure your child can breathe and nothing is bending at any weird angles. Also, do your best to soothe. Speak in a soft voice; tell him you love him, that it's going to be okay, or sing a lullaby she likes. Don't expect an immediate response, but it will sink in and they will feel more confident in their bond with you. When your child (FINALLY!!!!!!) begins to calm down (AFTER 19 HOURS OF SCREAMING!!!!!), reinforce the fact that everything is okay, and s/he shouldn't feel bad about going off.
Tantrums can be scary, especially for parents of Autistic children. But when you know how to handle them, you can get through them with at least some of your sanity intact.