Super Mutant Behemoth, Anyone?

There's always a random element to it… even some of my dwellers with better gear than yours struggle with lesser enemies, such as Enclave Deathclaw or Super Mutants.

According to this writeup, the formula that determines success in an encounter is as follows:

level / 2 + random(0, stat) + random(0,stat) + random(damage min, damage max)

Where stat is the stat the encounter tests against (each enemy is "weak" to single stat). The calculated value of above formula is then compared to a difficulty value of the encounter (if yours is higher, you win the encounter).

I wasn't able to find which stat SMBs are weak to though.

So overall, an armor with +2 to four different stats is really useful, as it raises your chances to win several different encounters. +2 to Endurance is an additional benefit, as it will make your dweller immune to radiation (Piper's armor comes to mind).

The only thing you can do to improve your chances of beating SMBs is to get a better weapon and maybe a Pet that does +X to damage.