Original Shazam is pretty tough to beat, but I don't think Billy would be able to beat him since he's not very experienced yet. Actually, would the wisdom of Solomon make him just as experienced? I don't know.
no offense to strange but with or with out prep shazam will crushed him. Shazam is a member of the quintessence a group of five omnipotent beings as they are described as in dcu. here are shazam powers
Magically bestowed superhuman strength, speed, stamina and courage.
Invulnerability
Nigh-omnipotent mastery of magic and mysticism.
Omnipresent and omniscient
Telepathy plus over ten thousand years experience
This thread is a dupe, but I'll say the same thing I said the last time.
Strange's "base-level" power state is less than that of Shazam hanging out on his throne. Strange could become like that if he wanted to, but he prefers to live more simply in the Villiage.
Strange can power up into the stratosphere at the drop of a hat though. If Shazam were to jump him while he's having lunch Stephen would probably go down, but if they talked smack before they rumbled expect them to be more or less equals by the time the fight started in earnest.
Stranges ace-in-the-hole against more powerful beings is always to steal their power and use it against them, and he's done that more than once to bigger fish than Shazam.
Dco also has millenia of experience. While not as many thousands of years as Shazam, but he did spend 5000 years fighting a war for the Vishanti. I'd call a year of combat experience more significant than several years of hanging out on the Rock fending off the occasional invasion or menace.
__________________
...You are Number Six. Respect Popeye