Dortmund will win no doubt!But it will be a tough and close match. I expect a 2-1 victory for Borussia.
But Dortmund not in good form and playing all match to end with draw so that i think Mainz has chance to win this match or most possible draw than double chance home or draw @2.54 will be good pick for this match.
I also don't think Dortmund will win this match as i also lost my previous on it all match even with huge underdog surprisingly ended with draw so that i also will agree with you bet on double chance home or draw is very good pick for this match.
I also think Mainz can win this match but Dortmund can make comeback at any time so that i will not take any risk against Dortmund will go for both teams to score has low odds but safe for this match.