  We present the results from the timing and spectral study of Mrk\,421 based
mainly on the \emph{Swift} data in the X-ray energy range obtained during the
time interval 2015\,December--2018\,April. The most extreme X-ray flaring
activity on the long-term, daily and intraday timescales was observed during
the 2-month period which started in 2017\,December when the 0.3--10\,keV flux
exceeded a level of 5$\times$10$^{-9}$erg\,cm$^{-2}$s$^{-1}$, recorded only
twice previously. While the TeV-band and X-ray variabilities mostly were
correlated, the source often varied in a complex manner in the MeV--GeV and
radio--UV energy ranges, indicating that the multifrequency emission of
Mrk\,421 could not be always generated in a single zone. The longer-term flares
at X-rays and $\gamma$-rays showed a lognormal character, possibly indicating a
variability imprint of the accretion disk onto the jet. A vast majority of the
0.3--10\,keV spectra were consistent with the log-parabolic model, showing
relatively low spectral curvature and correlations between the different
spectral parameters, predicted in the case of the first and second-order Fermi
processes. The position of the synchrotron spectral energy distribution (SED)
peak showed an extreme variability on diverse timescales between the energies
$E_{\rm p}$$<$0.1\,keV and $E_{\rm p}$$>$15\,keV, with 15\% of the spectra
peaking at hard X-rays and was related to the peak height as $S_{\rm
p}$$\varpropto$$E^{\alpha}_{\rm p}$ with $\alpha$$\sim$0.6, which is expected
for the transition from Kraichnan-type turbulence into the \textquotedblleft
hard-sphere\textquotedblright~ one. The 0.3--300\,GeV spectra showed the
features of the hadronic contribution, jet-star interaction and upscatter in
the Klein-Nishina regime in different time intervals.
