Create a custom player and add them to your opponent's team. Then, choose "Quick Tactics" and all of your opponent's players to attack. Place an arrow from the goalkeeper to the striker and you should win easily.
Fire your old assistant manager and acquire the best one possible by going to "Search" on the tree menu, then clicking "Staff". Make a plan on where you want to come in the season and in the next three seasons, then decide who you are going to release and your main transfer targets for that season. Try ti get the teams morale up and work on that.