If I'm not wrong, the sentence
"I want to help you with XYZ."
means, you have a problem XYZ and I am willing to help you solve that problem of yours. Right?
Now, when I have certain expertise and I want to help you solve your problem using my expertise, how do I put the whole thing in a sentence properly?
"I want to help you with my expertise." doesn't sound right.
with can introduce the tool or implement (broadly understood) that you are using, or the thing or situation that requires help:
I want to help you with that fallen tree with my chain saw.
I want to help you with your startup company with my business acumen.
They helped the town with its recovery after the earthquake with portable shelters and tankers carrying fresh water.
Perhaps a phrase along the lines of:
Using my expertise, I want to help you.
With my expertise, I want to help you.
I would like to offer my expertise to help you.
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.
