The part of the sentence "I'll be glad to assist" is correct and usable in written English. You can use this expression when you want to offer assistance to someone. For example, "Sir, if you need any help, I'll be glad to assist.".
"Our friendly staff will be glad to assist you in finding that perfect toy or gift".
If you need directions or someone to follow you over there, we will be glad to assist".
Don't be shy or embarrassed––few people want to sit at a wobbly table and most waitstaff will be glad to assist you.
A.: Yes, I'll be glad to.
Wendell Potter: I'll be glad to.
I will be glad to wear one.
I will be glad to.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK