I used a...
BLACK+DECKER 400 V Scorpion Electric Saw with 3 Blades and 10mm Stroke Length, KS890ECN-GB
...to take out two whole apple trees including a 6" diameter trunk, but it is corded, so needs to be near power!
I've also used one of these...
BLACK+DECKER BDCJS20C 20V MAX Jigsaw with Battery and Charger
...but it is a bit ungainly!