\documentclass{article}
\RequirePackage{geometry}
\geometry{a0paper}
\usepackage{xcolor,array}
\usepackage{tabularx}
\usepackage{booktabs}
\newcolumntype{C}{>{\centering\arraybackslash}X}
\begin{document}
\rule\textwidth{3mm}
% 1360pt works
\begin{minipage}{1380pt}
\tracingtabularx
\setlength{\tabcolsep}{0pt}
\begin{tabularx}{\linewidth}{
p{4.2cm}
|p{8.9cm}
*{11}{|C}
}
\toprule
&
&
\multicolumn{2}{c}{huh}
& A
& B
& C
& D
& E
& F
& \shortstack[c]{G\\H}
& \shortstack[c]{I\\J}
& K
\\
\midrule