While I have had limited experence with him, I will continue to update this guide as I learn more.

Blue Beetle is a great, well rounded champ. His assassin mode works well with jungles, and can help catch up if you are behind on levels/coins. Unfortunatly, you can not unlock it until level 6. Here's some more information:

Stance Mechanics
Once you reach level 6 you may level up your ult, unlocking assassin stance. When you do this you will find that all your skills (except your passive) are level 1. Your Q, W, and E's levels depend on the level of your Ult. (So if your Ult is at level 2 they will all be level 2). 

Laning in Gotham Divided
When playing Gotham Divided, I feel it may be best to lane with a partner. Usually this is done on the bottom lane. By laning with a partner you will be more likely to score kills due to the fact you will be in mini-team fights. You will also be able to take advantage of some of the nearby jungles (granted your partner can hold the line, and your team's jungler does not mind). Conversly you could take the top lane, but you will not be able to jungle as often. And due to the nature of assassins, soloing a lane is not always reccomended.

Using Reach Directive/Disrupt to Your Advantage
The way BB's passive works is quite interesting. When in Marksman stance, after activating a skill your next attack will do 30 True Damage. BUT: It will only do 30 if the target is at full health. If the target is missing health, 1% of what's missing will reduce that damage by 1% (So if a target is missing 200 health then you will have 2 true damage subtracted from your 30. This means you would then do 28 true damage. Conversly if they are missing 1000 health you will deal 30 -10 True Damage (which would be 20)). 
HOWEVER: When in Assassin stance the OPPOSITE is true. For every 1% of the target's REMAINING health you will loose 1% of your damage. Thus if your target is a tank with 2800 health and is at full health, you will then do only 2 True Damage. Pretty useless. So wait until your target is lower on health to go into Assassin Stance.

This means that if you can range someone to less than half health, you can effectivly leap in for the kill as an assassin. This works beautiflly in game. When in Assassin stance you gain 20 move speed and can use Mandible Taser to slow your prey to a halt.


Conclusion
Overall, Blue Beetle is a great assassin. You could almost think of him as a ranged assassin, someone who can wear down their target without taking a hit then jump in for the kill. You could try to play him as just a marksman, but if you do you are missing out on a vital mechanic of this Champion. 


Misc Tips

If you are in danger of death and need to escape, use Stick and Move then go into Assassin stance. The +20 move speed will help you make a hasty get-a-way.

Use your opponent's jungles often! They will only make you stronger, and you can get away pretty easily with the +20 move speed bonus + Mandible Taser

Lifesteal is never a bad thing. I reccomend getting some early on so you can stay out in the field longer (but ballance this carefully with your damage. Wouldn't want to be underpowered in fights ).

Attack Speed is your friend. Remember, you are doubling as a Marksman AND Assassin. To me, that makes the Mega Rod and Deathstroke's Claymore essential to this Champion.

If have fallen behind the other team level-wise, try purchasing the Maruder Knife. You can use it to jungle faster, plus if you get the Lifesteal that can help. (NOTE: I have yet to test this in game, but it should work in theory).