The obvious easy one shot tank kills are artillery vs arty, but many Tank Destroyers or any other tank are capable of one shot kills, you just need to either set up the tank you are using to go for this cheevo. Also pick and choose what targets are easily destroyed with one shell. The right ammo may help with getting easy one shot kills. you can use HE(high explosive) rounds to get easy kills if you penetrate the target or hit them at the right weak spot ie ammo rack or set them on fire.
Basic rule of thumb. Look at your tank garage and cross reference with any online guides to see if any of your tanks have high alpha damage, if not do some research prior to purchasing or working up a tank tech tree to get a tank solely for this cheevo. Also learn the weakspots, a one shot kill may not only be a direct dmg done by a pen shot, but setting the tank on fire and or hitting the ammo rack.
A few examples of some tanks( non arty)which are capable of one shot kills due to their ridiculous alpha damage for the tier they are played with.
* just remember if you are high tier, many of the lower tiers in that match will be easy one shot kills.
T18 USA Tier II TD, usually matched up in a tier I/II lobby
T82 USA Tier III TD, usually matched up in a tier II/III lobby
ISU152 Derp gun (use the HE option)
KV2 Russian Tier VI Heavy use the Derp gun
Jag PZ E100 German Tier X TD
UC 2 PDR UK tier II TD
FV215b 183 the deathstar ..highest alpha in game for a non arty.
If you need to buy a tank for this cheevo, try picking the lowest tier possible to save on the grinding. unles you are stacking other cheevos.