I won't pretend to be a pro when it comes to sideboarding, so what I've listed below are suggestions of what's good to take out and what's best to put in against each of the common decks. I won't offer quantities anymore, because it can be very situational depending on your opponent's build.

Vs. Ramunap Red

Outs: Spell Pierce, Rhonas the Indomitable

Ins: Vizier of Many Faces, Sorcerous Spyglass, Crushing Canopy, Shapers' Sanctuary, Deathgorge Scavenger

Spell Pierce won't put in enough work against their ridiculous amount of shocks and bolts, so you're also going to have a hard time keeping Rhonas the Indomitable active.

Vizier of Many Faces can copy their Hazoret, or you can shut them down with Sorcerous Spyglass.

Deathgorge Scavenger is a great help in getting rid of an Earthshaker Khenra in the yard.

Vs. Temur Energy

Outs: One With the Wind, Spell Pierce, Bristling Hydra

Ins: Vizier of Many Faces, Crushing Canopy, Negate, Sorcerous Spyglass

This is a matchup where you are guaranteed to run into Glorybringers, so you'll want to either bring in Sorcerous Spyglass to turn off their Hydras - in which case you might side out your own to make room - or maybe Vizier of Many Faces to copy their Glorybringers/Hydras

Negate is good if they're running multiple Chandras, though you might be better off taking Crushing Canopy to kill their Glorybringers.

Vs. GB/Sultai Energy

Outs: Bristling Hydra, Spell Pierce

Ins: Vizier of Many Faces, Shapers' Sanctuary

Sultai energy tends to run Hostage Taker, so you don't want to risk them stealing a Hydra from you. You're best off being able to copy their The Scarab God or their Hostage Taker. Sanctuary gives you a draw whenever they target your creatures, and then I believe you can Blossoming Defense to save them after the draw trigger goes on the stack. It also provides crazy draw if they repeatedly use Walking Ballista to kill something, since it counts as multiple activations.

Vs. UW Approach

Outs: Bristling Hydra, One With the Wind, Spell Pierce

Ins: Negate, Crushing Canopy, Nissa, Steward of Elements, Sorcerous Spyglass

Against this deck, you want to be prepared for them to drop a Solemnity out of nowhere, so Crushing Canopy is a hedge against that, though it's not likely to be used. Negate is far more valuable, shutting off their main win-con long enough for you to kill them...hopefully. Nissa, Steward of Elements allows you to put them on a 2 turn clock, Sorcerous Spyglass turns off their Gideon of the Trials.

Vs. UB Core Control

Outs: Bristling Hydra, One With the Wind, Spell Pierce

Ins: Sorcerous Spyglass, Negate, Nissa, Steward of Elements, Vizier of Many Faces, Deathgorge Scavenger

Against UB Control cores, you're risking The Scarab God reanimating your stuff, which makes everything a major threat if they kill it. You can't overcommit to the board, so you want to make sure they can't steal a Hydra at least.

You can happily bring in Spyglass to turn off their Scarab God and any planeswalkers they might run. Vizier of Many Faces lets you blank their Scarab God and gives you inevitability off your own creatures.

Nissa, Steward of Elements is potentially useful in any control match up since at the worst, you'll force them to Disallow her. Just don't try to cast her for an instant 6 loyalty.

Vs. UR Core Control

Outs: Bristling Hydra, One With the Wind, Spell Pierce

Ins: Negate, Sorcerous Spyglass, Vizier of Many Faces, Nissa, Steward of Elements, Deathgorge Scavenger

Hydras can be a liability or they can be absolutely brutal and it hinges on whether they have a black splash for Scarab God or not.

Due to the threat of Abrades, this is another matchup where you might consider going on a "grind them out" plan instead of committing to the Pummeler kill.

Vs. Gift of the God Pharaoh Variants

Outs: Bristling Hydra, Spell Pierce

Ins: Negate, Nissa, Steward of Elements, Crushing Canopy, Deathgorge Scavenger

While it may seem strange to advocate the boarding of Hydras, the GPG decks have taken to running a lot of fliers, so you're not going to blank them with a double pumped Hydra. You would have to stick One With the Wind on it. You're likely best off going on the offensive, using Negate to stop them resolving GPG or Refurbishing it, taking targets out of the yard with Deathgorge Scavenger and falling back on Crushing Canopy to destroy flying threats so you can go in for a fast lethal.

Vs. Anointed Procession Variants

Outs: Bristling Hydra

Ins: Crushing Canopy, Negate, Sorcerous Spyglass

Bristling Hydra won't do much for you here. They go wide too fast. You need to be able to just keep slamming in with low cost creatures who fly or trample with potentially a 9 energy Pummeler.

Crushing Canopy can slow them down by taking out Anointed Procession or you can Negate it. Spyglass is a hedge bet, allowing you to turn off Gideon of the Trials and the flipped Legion's Landing

Vs. Vehicles Variants

Outs: Bristling Hydra, Rhonas the Indomitable

Ins: Sorcerous Spyglass, Crushing Canopy, Negate

Spyglass turns off their Heart of Kiran entirely. You probably won't have the time to get to and stick a Bristling Hydra which is why I advocate the boarding out of it. They run a fairly decent amount of removal, so Rhonas the Indomitable can easily be stranded.