There are all sorts of ways to fight using partial transfiguration. Many of the obvious ones mentioned so far are too slow (tunneling through the ground, nerve gas), suicidal (micro black holes, antimatter, unununium, critical mass of enriched uranium, nitroglycerin, etc.) or too complicated.

only a fool would attempt a plot that was as complicated as possible, the real limit was two.

On the other hand, Dumbledore took the shotgun approach to plotting. Let's take the best of both worlds.

Then the obvious answer is chain lightning! Transfigure the air into ions in a path connecting the Death Eater's necks, including LV. This will likely go unnoticed since air is invisible. It might take a few seconds, but Harry can stall in Parseltongue. Then transfigure a cross section of the middle Death Eater's spinal cord into pure electrons. You should be able to generate considerable voltage with minimal mass/volume/time. Repeat for the other Death Eaters. This gives us the possible instantaneous effects of:

• Blinding flash/deafening thunder
• Concussive shock
• Electrocution
• Combustion
• Death

To all present Death Eaters simultaneously, while not killing Harry outright, plus

• Paralysis
• Death

To the targeted middle Death Eater. Suffering even one of these effects will disable the other Death Eaters long enough to repeat the procedure. The initial strike will reinforce the ion channels, Harry won't have to rebuild them.

Of course I'm assuming partial transfiguration doesn't require Harry to actually aim his wand at the target, but is limited in size. I'm also assuming he can transfigure air, which is not clear from the story so far, as others have mentioned. But really, air is just atoms like everything else. A partial transfiguration should work. Aim should not be required. Contact should not be required. Just partially transfigure a ray of air into air in the intended direction, and terminating with the intended effect.