"indeed strong" is correct and can be used in written English.
It is used to emphasize an opinion or statement, such as in the sentence, "Their commitment to the cause is indeed strong."
Penelope is indeed strong and true: she has kept the suitors at bay for a decade.
The Malty-Melty is indeed strong, and grainy, from the malt.
The low turnout reflected not just apathy and fatalism, which are indeed strong.
Indeed, strong FDI is one of the few positive trends in Ireland.
The force is indeed strong with this one, but there are plenty of forces conspiring against it.
Indeed, so strong was their presence, that Courtlandt Avenue came to be known as "Dutch Broadway".
Scientists say their statistical evidence for an increase of weather extremes is indeed strongest when it comes to heat waves.
Indeed, strong-willed entrepreneurs harboring hopes of Gatesian wealth are loath to relinquish control of companies they started from scratch.
Fingerprint biometrics are indeed stronger because they cannot be easily faked, altered or stolen.
In this configuration, ideological polarization is indeed stronger and more stable over time.
