As you're 3.5 you might be in a not-so-good situation. I don't know about your country's rating system and how you play, but I can give a few hints.
Slices are low powered and if not executed correctly very easy to attack them. When you get the chance, run around to the forehand and attack. Shorten the points as you can't withstand the long rallies physically.
So shorten the points - attack his serve, his weaker slices, run around to the forehand and what's very important: get to the net. He can't get good passing shots with slices, so after an attacking shot at the service line area, transfer to the net for an easy volley or overhead.
Move him to side to side, put him on the defensive. Avoid long rallies that can exhaust you: if you see the opportunity, close the point out. If you get him moving, you can get him exhausted, thus he will make more UEs and the match will be easier for you to win.
And for the last thing I'd say is the serve - hit in solid first serves: flat, but vary it from backhand to forehand and body. If you get a good 1st serve, attack the return and if possible get to the net. Get your 1st serve percentage up.
I wish you luck!