Defending Giannis 1-1 is suicidal.
There's only been three proven ways to guard Giannis with
some success:
1) The OKC way: assemble a crew of long athletic guys that look as close as possible to Giannis (Grant/PG/Noel), add a monster of a C like Adams, and keep two of them on Giannis when he gets anywhere near the basket. Assume a low defensive stance to try to keep him out of dunking distance, don't foul, and when he tries a lay up or hook shot go straight up and try to block him.
In other words just play good, honest D and try to collectively out-physical him. No trickery.
Chances of success: average to good (see away game @ OKC this year)
Disadvantages: you need those type of lengthy athletic defenders that Boston obviously doesn't have.
2) Boston series game 1: Get back on D super fast, I mean everybody literally sprint back every time, and set your D up. Pick up Giannis close to mid court with a big guy who can move a little (Horford). Add a big wing on each side with their arms open, focusing mainly on Giannis and sagging a little bit off the players they're guarding. When Giannis sees that wall he will likely pass the ball, and that's already a small victory. Eventually Giannis will try to split that wall (usually from the right), which makes him vulnerable to swipes from the left wing. Failing the swipe the left wing follows him all the way to the rim along with Horford, swiping and hacking throughout. Horford keeps and arm on him at all times to kill his momentum and making his second step very slow.
Chances of success: average, Giannis will probably still rape you eventually.
Disadvantages: very physically taxing for the D. Not sure if Boston can actually do it for many games, which is maybe why they gave up on it after G1. Also, you open yourself up to many foul calls depending on the typical NBA ref's bipolar mood swings.
3) The fainting goat. Just fall down man, as soon as he gets anywhere near you, just faint. Convulsions, involuntary spasms and pissing yourself also help.
Chances of success: small, it's a pretty desperate move.
Disadvantages: none, it's also very funny to watch