Just to be very clear:
When you say "I want to help you with...," the phrase that follows is what the person needs help with, not what you are going to use to help them.
Your last sentence doesn't make sense because the other person doesn't possess your expertise, or need help with it. They need help stemming FROM the expertise, not WITH it.
Like the top answer says, it would be best to say "Using my expertise, I want to help you." Then you can add "with [thing they need help with]"
You can also put the tool you're using (your expertise) after the thing they need help with, like so:
"I want to help you with [your problem], using my expertise."
The most important thing is to keep the core phrase intact. "I want to help you with..." should always be followed by what they need help with.