A few thoughts....

Short backswing/follow-through slash vs. big thrust motion

- a chainsaw is not like a two-handed sword in that it doesn't require overall momentum to **slash**... for a slashing movement with a sword, some backswing is required, but with a chainsaw the blade can be thrust/twisted more directly at the target, so as defender you can't wait for an overt backswing to slip in on the attacker, nor expect much of a follow through after some kind of slash in which to close

- it does still take a major effort to **thrust** the weapon forwards, so you can expect more telegraphing of and recovery time from such a movement, which means it's a better opportunity to counter attack

Dominant hand and range of wrist motion

- Let's say the attacker's right handed - they're going to find it natural to slash diagonally from their upper right towards their lower left.  It'll be hard/unnatural for them to start any kind of slash from their left.  Once their right arm's straight and left elbow bent in against their ribs, they can't turn the blade any more.

- to exploit this fully, you want to attack them while their left foot is landing forwards - back away while observing the timing of their steps, then step in as their left foot comes forwards.  The longer their stride the more they'll be twisted away from you.  If you feint as though moving to their open side they're likely to adjust to step their left foot further to their right too.  Good options then include:

  - sliding to the outside of their left/front foot (typical aikido move) so they have to twist the chainsaw across their own body to reach it towards you, and their left arm will get in the way of rotating the blade into you.  As you slide past, use a ridge hand / reverse knife hand / arm bar / palm etc across their throat/face, or a left low kick to their front knee

  - hard low kick to their front knee then retreat quickly

In desperation

- be creative with other objects you might use to your advantage - e.g. flicking a belt at their hands or face while keeping out of reach of the chainsaw; don't rely on this for long - it's a high-stakes game - once you see they're distracted by it seek an opportunity to do something more decisive

- the chain rotates around the edge of the saw, so there's some small potential to deflect the blade through contact inside the chain - if you somehow get stuck with the thing being thrust at your head or chest and have no ability to dodge, a absolute last-ditch do-or-die option is deflecting it with a fingertip or flatfist strike to the area inside the chain, but if you can reach further to get past the chain deflect it there of course

It'd be really interesting to have someone hold a chainsaw while someone else struck across the blade with an old leather belt, coat etc to see how strong a sideways tug the holder experienced before the belt/coat disintegrated - they might affect a useful momentary deflection.  Sadly, I don't currently own a chainsaw.