Personally, I don't mind lesnar as champion because even if he's not always there, when he is, he makes the title actually feel like a world title. He did more to elevate the title to a world title status in the few months he had the belt then kevin owens did with his whole run.
Lesnar feels like a world champion and his booking as been perfect since winning the belt and the longer he holds the belt the better the belt will be.
As far as how you book guys against him, it depend on who he's facing, if he wrestles guys like reigns, joe or strowman then you book him to have a competitive match with them because it's believable that these guys can go toe to toe with lesnar. While if you put him with smaller guys like rollins or balor, then you have to book those matches as one sided as possible because again believability is important and if you book a 50/50 match between a super heavyweight and a cruiserweight. So you need to book them as been underdog with lesnar selling the less amount of offense.
Since lesnar's contract ends at wrestlemania, it makes total sense to keep him as champion until the end of his contract, that way whoever beat him at mania become a bonifide main event star, the same way that they did with reigns last year when he beat taker at mania.