The main weakness for many exponents in the martial arts when facing multiple opponents is their body cannot hold together under acceleration, particularly when they encounter the resistance forces from members of the group. The result is they get stalled on the first opponent and this gives the rest of the group time to take them down.
You have to be able to keep the body together and acting as
one unit, if you are going to be able to focus the energy generated by your bursts of acceleration. When you are in contact with an opponent you cannot split
your energy.
The core or mid-region is an area that needs the correct kind of training. People train their core with exercises such as push ups, sit ups, yoga, pilates, etc. You need to distinguish however between training to strengthen your core and training to make a 'working core' that can function under accelerated movement.
A working core will ensure better coordination between the torso and the legs. This will enable you to develop better strategies for dealing with multiple attackers.