Awesome game. One of the first I ever played, but I still enjoy playing it to this day. Just a fun game overall.
To beat the game, all you have to do is run through each of the 5 levels in each Warp Room, collect the crystal, and then beat the boss in between the Warp Rooms. Simple as that. But, for an even bigger challenge, you can try to collect all 25 crystals, all 37 gems, and all 5 colored gems. The colored gems are special because they give you paths that you couldn't normally reach without the right gem. This helps you collect every box in the game, and then you get another gem.
The bosses are fairly simple in this game, it's not meant to be hard, it's just a game you play for fun. In between each room, you have to beat a different boss, but most of them are from the original Crash Bandicoot, so nothing too new.
Overall, Crash Bandicoot 2: Cortex Strikes Back is a great game, and even though now it is really old, it is still fun. If you are tired of playing difficult adventure games, and you want something simple, but still fun, thenn this is an excellent choice.