I'm afraid no-one will be able to answer this as genetically your pup could inherit a mix of behavioural traits from both breeds i.e. a Boxers manic bounciness coupled with a Mastiffs guarding tendencies - just as physically you will have no idea how he/she will turn out. To be frank these crosses are a BAD idea and folk should not encourage this kind of irresponsible breeding by buying such pups.
Sorry to be so negative but this is something I feel very strongly about - you should be asking yourself what kind of characteristics you want in a dog to fit your lifestyle and then look for a pedigree breed that will guarantee you the kind of dog that is ideal for you.
I agree with milor- it's really pot-luck with cross-breeds as to what sort of temperament they will have. I own a cross-breed rescue dog; God knows what breeds are in there, but certainly a herding breed of some sort and something with boundless energy and no fear, but also (down to luck rather than judgement of whoever bred her) a faultless temperament.
I also own a pedigree cocker whom I bought from a breeder; I chose this breed because (after doing some research on several possible breed options) the breed typically suited my lifestyle and requirements; and that particular breeder because her line had proven temperament (ALL the dogs I've met or know from my cocker's breeding line have a very similar, easygoing, sweet nature with no aggressive tendencies.) I got exactly the kind of dog I wanted, expected and paid for. While some of a dog's temperament will be down to how you raise it from a pup, some of it is bred and it makes sense to choose a dog that is well-bred temperament-wise, and one whose breed characteristics meet your requirements; at least then you have a good basis to build from, and pretty much know what to expect.
It might be a good idea to do some research on Boxers and Bull Mastiffs. Read the breed standards, look at websites, but don't expect your dog to conform in any way to either breed in particular. Read the bad stuff too- all breeds can have traits that are less than desirable; and forewarned is forearmed.
I looked up both breeds before agreein to buy the pup as with any breed x or pedigree they can be nasty or nice I was just wondering if any one had any experance with this type of x I know it all varies on the indvidual dog The information I havee read both breeds sound good for my situation