I have a Mi'Quote tank which makes life difficult for maintaining aggro but here is best new player(me) advice:

Once you use Shield Lob and establish pull run a little toward the encroaching group and hit flash, then pull back to give yourself some breathing room.

As you step back you should start to see which enemies are taking the most damage from your pull list. Immediately target that enemy that is taking the most damage with Shield Lob and go to town.

If the damage is spread out more you just have to alternate repeated use of flash while doing your full combo on the weakest monster.

If it is a slower group you can buy yourself time by stunning a couple monsters to start and then flashing again until a main target emerges.

Obviously, as noted above, top gear for STR and Shield Oath will be essential at higher levels; I've tried with Sword Oath and it was a mad house.

I haven't run with two high-level ninjas so I'll have to get back to you on how that works out ;n_n