As for general counters and tactics:
-Watch for Purging Flames and Judge’s Intervention – both of these skills can start off a burn burst (sometimes together), and reacting effectively to them will put you at a significant advantage for the next ~30 seconds. When you see Purging Flames go down, you can expect to see the Guardian pull out their Greatsword and quickly follow up with either Whirling Wrath, Leap of Faith, or Binding Blade, depending on the situation. WW can stack a huge amount of burns with PF thanks to Burning Bolts from the Fire field, while LoF gives the Guardian a Fire Shield for a few seconds that will force you to back off. Binding Blade is another Whirl Finisher, but it’s mostly dangerous because it can pull people into PF (inflicting 3+ stacks of burn immediately) and set them up for a Whirling Wrath follow-up. Conserve some of your dodges, stun breaks, and other defenses until you see that field go down in each fight. Even if you’re not inside of it, don’t let your guard down.
-When a burn Guardian uses JI, there’s a good chance that you’re going to immediately end up with 6-8 burns on you, since they’ll often start casting their attack (Zealot’s Defense, Zealot’s Fire, Whirling Wratch, etc.) before using JI and warp to you the moment it fires. It can be hard to completely avoid a burst from a well-timed JI, so keep your condition cleanses on hand. If possible, try to interrupt the Guardian’s momentum with blocks, teleports, or hard CC as soon as they warp in. JI is a gap-closer, a heal, and one of the Guardian’s two stun breaks, so getting us to waste it is a great way to gain the upper hand.
-If you don’t see a burn Guardian use Purging Flames during your first extended fight with them, they’re probably running Smite Condition instead. SC gives the Guardian more sustain against direct damage, but more importantly, it changes the way that they burst. SC grants the Guardian Fury and can crit for up to 3k with a Carrion build, and with the Smiter’s Boon trait, they’ll automatically cast SC whenever they use their heal. Guardians using Shelter can burn you if you hit them during their heal, and they’ll proc a second AoE burn if they’re running Balthazar runes (which most do). All of this comes out instantaneously, so if you’re starting to get the Guardian low, you need to watch for a SC+Shelter counter-burst. This will give them a 2s continuous block, heal them for 8k, cleanse two of their conditions, potentially give them 2 stacks of might, hit you for up to 3-6k damage, and put 2-3 stacks of burning on you. This is also an AoE effect, making it especially dangerous if you have multiple allies nearby. Timed appropriately, it can end a teamfight before you have a chance to react.
On a Warrior: As others have said, CC and Berserker’s stance/other cleansing are the key, and the Mace/Shield + GS build is probably more equipped to deal with a burn Guardian than the Hammer/GS build (I don’t recall seeing a Shout Warrior yet in this meta, so I can’t speak for them). Take Brawler’s Recovery and swap weapons often, and be prepared to counter JI with a block (e.g. Shield Stance), stun, or dodge/evade (e.g. Whirlwind attack). Once JI is gone, we’ve only got Contemplation of Purity left to break stuns (and vice-versa), so if you can use a short CC to bait out one of these stun breaks, you can put a lot of pressure on us. Popping Berserker’s Stance early and playing aggressively can go a long way.
On a Mesmer: As mentioned above, kite and CC from a distance. Do NOT stand near your clones, regardless of how far you are from the Guardian or whether you’re stealthed. Burn Guardians will cleave through them in 1-2 hits from a Sword or GS, and your clones feed us VoJ procs for Permeating Wrath. A Guardian can use Flashing Blade, Leap of Faith, and/or JI to rapidly close the distance to you, and being in a crowd of bodies during a WW or Sword auto chain will melt you even if the Guardian is swinging at the wrong target. Don’t give them the opportunity. Be especially careful of Guardians with Smite Condition, since they can wipe most of your clones out instantly every 16 seconds.
If you’re running any type of interrupt build, try to force the Guardian’s stunbreaks with your CC’s, but be aware that the Guardian may still be able to block part of your burst with Shelter/Renewed Focus. Don’t over-commit to a burst until you know that the Guardian has depleted their defenses. With PU, just keep stealthed and at a distance while you wear the Guardian down. If you’re using conditions, it may be in your interest to space them out. We can flip a condition burst into a surge of boons via Contemplation of Purity, or use Purging Flames to cleanse and reduce incoming condi duration for several seconds, but our cleansing over time is weaker.
I’m out of time, but I tried to touch on some points that hadn’t been covered. Hope this helps!
