documentclass[12pt]{article}
usepackage{amsmath,amsthm}
usepackage{mathtools}
usepackage{tabularx}
usepackage{multirow}

begin{document}

begin{table}[h!]
begin{tabular}{|c|c|c|r|r|r|r|r|r|r|r|}
hline
multicolumn{2}{|c|}{multirow{2}{*}{Heading 1}} & multicolumn{9}    {c|}{$theta$} \ cline{3-11}
multicolumn{2}{|c|}{} & multicolumn{3}{c|}{1} & multicolumn{3}{c|}{1.5}                                                        & multicolumn{3}{c|}{2}\ hline multicolumn{2}{|c|}{$alphaRightarrow$} & 0.1 & multicolumn{1}{c|}{0.2} & multicolumn{1}{c|}{0.3} & multicolumn{1}{c|}{0.1} & multicolumn{1}{c|}{0.2} & multicolumn{1}{c|}{0.3} & multicolumn{1}{c|}{0.1} & multicolumn{1}{c|}{0.2} & multicolumn{1}{c|}{0.3} \ hline
multirow{3}{*}{x1/x2} & 1/3 & multicolumn{1}{r|}{34.38%} & 16.95% & 9.74% & 87.58% & 36.25%  & 45.85% & 30.10% & 67.21% & 59.25% \ cline{2-11}
& 1 & multicolumn{1}{r|}{61.52%} & 79.96%  & 68.17% & 75.50%  & 36.18% & 48.46% & 32.88% & 29.26% & 9.76% \ cline{2-11}
& 3 & multicolumn{1}{r|}{53.70%} & 68.99% & 85.41%  & 37.80% & 85.88% & 25.70%  & 32.32% & 93.95% & 81.96% \ hline
end{tabular}
end{table}
end{document}

documentclass[12pt]{article}
usepackage{amsmath,amsthm}% not used
usepackage{mathtools}% not used
usepackage{tabularx}% not used
usepackage{multirow}
usepackage{graphics}% for rotatebox
usepackage{wrapfig}
usepackage{showframe}% debugging tool
usepackage{lipsum}% debugging tool

begin{document}

begin{wraptable}{r}{0pt}
rotatebox{90}{begin{minipage}{dimexprtextheight-2intextsep}
caption{Test}
centering
begin{tabular}{|c|c|c|r|r|r|r|r|r|r|r|}
hline
multicolumn{2}{|c|}{multirow{2}{*}{Heading 1}} & multicolumn{9}    {c|}{$theta$} \ cline{3-11}
multicolumn{2}{|c|}{} & multicolumn{3}{c|}{1} & multicolumn{3}{c|}{1.5}                                                        & multicolumn{3}{c|}{2}\ hline multicolumn{2}{|c|}{$alphaRightarrow$} & 0.1 & multicolumn{1}{c|}{0.2} & multicolumn{1}{c|}{0.3} & multicolumn{1}{c|}{0.1} & multicolumn{1}{c|}{0.2} & multicolumn{1}{c|}{0.3} & multicolumn{1}{c|}{0.1} & multicolumn{1}{c|}{0.2} & multicolumn{1}{c|}{0.3} \ hline
multirow{3}{*}{x1/x2} & 1/3 & multicolumn{1}{r|}{34.38%} & 16.95% & 9.74% & 87.58% & 36.25%  & 45.85% & 30.10% & 67.21% & 59.25% \ cline{2-11}
& 1 & multicolumn{1}{r|}{61.52%} & 79.96%  & 68.17% & 75.50%  & 36.18% & 48.46% & 32.88% & 29.26% & 9.76% \ cline{2-11}
& 3 & multicolumn{1}{r|}{53.70%} & 68.99% & 85.41%  & 37.80% & 85.88% & 25.70%  & 32.32% & 93.95% & 81.96% \ hline
end{tabular}
end{minipage}}
end{wraptable}

lipsum[1-3]
end{document}