I have a table (code in the final) that is rotated 90 degrees counterclockwise. However, it is too wide. How can I solve that (fit to the width of (rotated) text block)? One solution could be to make the first 2 columns, multi-line columns but I'm not being succeeded in doing do.
\begin{sidewaystable}
\centering
\caption{My caption}
\label{my-label}
\begin{tabular}{ccccccccccccccccc}
\hline
\multicolumn{1}{|c|}{\multirow{3}{*}{Algorithm}} & \multicolumn{1}{c|}{\multirow{3}{*}{Prediction Method}} & \multicolumn{15}{c|}{PH (min)} \\ \cline{3-17}
\multicolumn{1}{|c|}{} & \multicolumn{1}{c|}{} & \multicolumn{3}{c|}{10} & \multicolumn{3}{c|}{20} & \multicolumn{3}{c|}{30} & \multicolumn{3}{c|}{45} & \multicolumn{3}{c|}{60} \\ \cline{3-17}
\multicolumn{1}{|c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{W} & \multicolumn{1}{c|}{RMSE} & \multicolumn{1}{c|}{$R_{adj}^2$} & \multicolumn{1}{c|}{W} & \multicolumn{1}{c|}{RMSE} & \multicolumn{1}{c|}{$R_{adj}^2$} & \multicolumn{1}{c|}{W} & \multicolumn{1}{c|}{RMSE} & \multicolumn{1}{c|}{$R_{adj}^2$} & \multicolumn{1}{c|}{W} & \multicolumn{1}{c|}{RMSE} & \multicolumn{1}{c|}{$R_{adj}^2$} & \multicolumn{1}{c|}{W} & \multicolumn{1}{c|}{RMSE} & \multicolumn{1}{c|}{Adj.~$r^2$} \\ \hline
\multicolumn{1}{|c|}{Naive} & \multicolumn{1}{c|}{-} & \multicolumn{1}{c|}{49} & \multicolumn{1}{c|}{7.43} & \multicolumn{1}{c|}{0.90} & \multicolumn{1}{c|}{49} & \multicolumn{1}{c|}{13.40} & \multicolumn{1}{c|}{0.69} & \multicolumn{1}{c|}{49} & \multicolumn{1}{c|}{17.82} & \multicolumn{1}{c|}{0.45} & \multicolumn{1}{c|}{49} & \multicolumn{1}{c|}{21.53} & \multicolumn{1}{c|}{0.16} & \multicolumn{1}{c|}{49} & \multicolumn{1}{c|}{22.75} & \multicolumn{1}{c|}{0.00} \\ \hline
\multicolumn{1}{|c|}{\multirow{2}{*}{Linear Regression}} & \multicolumn{1}{c|}{Direct} & \multicolumn{1}{c|}{34} & \multicolumn{1}{c|}{5.10} & \multicolumn{1}{c|}{0.96} & \multicolumn{1}{c|}{19} & \multicolumn{1}{c|}{11.03} & \multicolumn{1}{c|}{0.82} & \multicolumn{1}{c|}{19} & \multicolumn{1}{c|}{16.08} & \multicolumn{1}{c|}{0.62} & \multicolumn{1}{c|}{16} & \multicolumn{1}{c|}{20.87} & \multicolumn{1}{c|}{0.36} & \multicolumn{1}{c|}{40} & \multicolumn{1}{c|}{21.92} & \multicolumn{1}{c|}{0.16} \\ \cline{2-17}
\multicolumn{1}{|c|}{} & \multicolumn{1}{c|}{Iterative} & \multicolumn{1}{c|}{34} & \multicolumn{1}{c|}{5.10} & \multicolumn{1}{c|}{0.96} & \multicolumn{1}{c|}{19} & \multicolumn{1}{c|}{11.03} & \multicolumn{1}{c|}{0.82} & \multicolumn{1}{c|}{19} & \multicolumn{1}{c|}{16.00} & \multicolumn{1}{c|}{0.62} & \multicolumn{1}{c|}{37} & \multicolumn{1}{c|}{20.71} & \multicolumn{1}{c|}{0.30} & \multicolumn{1}{c|}{43} & \multicolumn{1}{c|}{21.58} & \multicolumn{1}{c|}{0.16} \\ \hline
\multicolumn{1}{|c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} & \multicolumn{1}{c|}{} \\ \hline
& & & & & & & & & & & & & & & & \\
\end{tabular}
\end{sidewaystable}
EDIT1: My preamble is:
\usepackage[usenames,dvipsnames,table]{xcolor}
\usepackage[square, numbers, comma, sort&compress]{natbib} % Use the natbib reference package - read up on this to edit the reference style; if you want text (e.g. Smith et al., 2012) for the in-text references (instead of numbers), remove 'numbers'
\hypersetup{urlcolor=blue, colorlinks=true} % Colors hyperlinks in blue - change to black if annoying
\title{\ttitle} % Defines the thesis title - don't touch this
%\usepackage[disable]{todonotes}
\usepackage[draft]{todonotes}
\usepackage[bottom]{footmisc}
\usepackage[export]{adjustbox}
\usepackage{multirow}
\usepackage{pgfgantt}
\usepackage{fixltx2e}
\usepackage{cals}
\usepackage{rotating}
\usepackage{array}
\usepackage[showframe, nomarginpar]{geometry}
\usepackage{graphics}
\usepackage{amsmath}
\usepackage{caption, makecell}
\setcellgapes{4pt}