Since the patch, Melee kills and hitting orange and green crates do not matter. Therefore using your gun is allowed.
So... Use your gun. And here's the best part. Your gun charges with time. So if you just sit there your gun will recharge.
Play 1-1 on easy. You still need to block a few, but groups close together of up to five can be killed with a full gun charge.
Know the numbers. And spawns.
Starts with 4. 1. 4. 4. 2 sets of 3 (if you stand back right against the red shield one set of 3 will spawn) wait to recharge and then kill the next set of 3. Then 5. Three will be in front wih two behind. Gun only usually kills the first three so be ready to block the other two or block one and Melee the other. Then 3. Then 5. wait for all to clump and run far back to where you turned and shoot. 4. 2. Stay at the top and finish.
Most important. Be patient for the gun to reload the whole amount. You never know when you will need that extra bullet.
I would like to thank Titeuf, who showed me that the gun recharged.
pS. On another play through there were a set of 3 followed by 5. I think I ran too far forward while triggering the first three. Try not to run too far forward and learn the trigger points.