There are several threads regarding this topic.
To make a long story short—yes, there will be a fair number of times when you face opponents with stronger creatures than yours. Most opponents are real players; some are bots. Due to the random selection of your 4 team members, it will often seem like the battles are “fixed.” That, however, is where strategy comes in.
By continuing to level up your team, replacing weak creatures with stronger ones, etc., you can do well in the battle arena. You will not always win battles in the arena–that’s just a fact. But you can do well and progress to the higher tiers given enough time and effort.
Best of luck, and happy hunting!