Boros Charm maybe?

and if you could decrease the number of lands you could insert Monastery Swiftspear

I wouldn't recommend running both Arena and Confidant at the same time. Vs aggro you're guaranteed to lose the race every single time.

I'm not sure what you would replace but I would really suggest adding cards like Manamorphose and Gitaxian Probe to your deck. Not only do the cards cost nothing and replace themselves (and proc Guttersnipe) but one helps round out your mana and the other gives you piece of mind. Just a thought. Nice deck though!

Guttersnipe is really not that good in burn. Why spend 3 mana on him when you could be playing 3 burn spells instead? When you play guttersnipe you basically time walk yourself. Also, confidant and arena do not belong in burn, they just don't, they do not work with the burn gameplan at all. I'd add in 4 Goblin guide, that's kinda mandatory in burn, and would probably add a Grim Lavamancer package by replacing the clifftop retreats and dragonskull summits with Arid Mesa's and Bloodstained Mires. Also you might want to take out the black entirely and go into green for atarkas command, and destructive revelry out of the board as that's what actual burn decks that are doing well in modern have been running. One last little nitpick, those colors are Mardu not Grixis.

4x Phyrexian Arena seems like a little much when you are running 4x Dark Confidant, I would suggest removing 2 or all of those and putting in 4x Lightning Helix which will help you stay alive longer with those and give you more removal / burn. Young Pyromancer would be worth looking into fitting in also. Another suggestion that I would make is to look into adding some more basic lands, right now you only have swamps and it's important to have at least 1 or 2 of each basic land in case you get Path to Exile or Ghost Quarter 'd (which will happen a lot in modern). Great deck though! gets +1 from me

Young Pyromancer > Goblin Guide in this instance in my opinion because you have already a lot of other stuff that you could be casting on turn 1 that is more effective if you are going to be bringing the pyromancer out on turn 2.

Demention the reason for 4x phyrexian arena is that i need to draw it or dark confidant. If i should remove any of those, it would be the dark confidant (because it's a lot easier to remove.) but after a lot of playtesting, i found out that i need all 8. I would much rather have Goblin Guide over Young Pyromancer because it's an amazing one-drop. If i have it in my opening hand it's usually atleast 6 dmg.

I see decks like this all over Tappedout, trying to incorporate slog into Burn. You don't want to play things like Dark Confidant or Phyrexian Arena in Burn as it deviates from the gameplan of getting your opponent from 20 to 0 as quickly as possible. Why spend turn 2 playing Confidant when you can deals 6 damage to them through 2x Bolts. Recommend Eidolon of the Great Revel and Lightning Helix.

NateJH but isn't it better to play Phyrexian Arena than to sit around and stare? After playtesting a lot, i seem to sit around with no hand if i don't play Phyrexian Arena.

I think Lightning Helix is a must have on that list, you lose a lot of life with Phyrexian Arena and BoB togheter.

What about Soulfire Grandmaster, test it with one or two, maybe she found a home hehe.

its better to top deck burn then another Phyrexian Arena.

try replacing the arena with Monastery Mentor or Soulfire Grand Master. it is too slow

I'm thinking those 4 MD Swamps should be Mountains instead? Perhaps you could do a 2/2 split between Arena and Lightning Helix to help offset the lifeloss from your Arena. Also Rending Volley should be a 2-of in your SB to help against Twin decks/random other W/U creatures, unless there is no twin in your meta. Smash to Smithereens is another good SB card in case you come across Affinity. If you only play casually then don't worry too much about your SB, play what you need.

Don't forget Rakdos Charm against twin.

Soulfire Grand Master turns every Lightning Bolt into a Lightning Helix and being able to cast to hand late game (if it gets to that point) is really helpful. I'd also have Crackling Doom in the side against hexproof or control finishers.

Vexing devils, soulfire grandmasters and Boros reckoners, are all good creatures for the burn decks. You have a tight enough curve to drop down to 18-17 lands. Also look for instant speed burn over sorcery speed burn.

now that i think about it though, this is supposed to be sub-40 so soulfire, devil, and reckoner don't really fit in here. that said, neither does Phyrexian Arena

With all of the life loss from Bob and Arena, I think adding Lightning Helix would really help you out race aggro decks. Plus it's just a fantastic burn spell on its own.

You're basically running nacatl burn without Wild Nacatl. Maybe try adding them instead of Mutagenic Growth? It usually comes in as a 3/3, and if they have the ability to deal with it G1 you can board it out. Also, depending on what your meta looks like I would drop 1 Lightning Helix for 1 more Searing Blaze. Cheers, it's a nice deck. Also, you probably already noticed, but you're 1 card short of a full deck.

CornX60 I can see that Wild Nacatl is a good card, but the reason i run Mutagenic Growth is because, if i have a Mutagenic Growth and a Goblin Guide or a Monastery Swiftspear on my starting hand, it's 4 damage first turn. I will look into getting Wild Nacatl in the deck though! About the Lightning Helix, i am running 2 Searing Blaze in my sideboard. If i play against a creature heavy deck, i will sideboard two Searing Blaze in and two Skullcrack out and vise versa.

my recommendation would be +4 Wild Nacatl, -1 Mutagenic Growth, -2 Rift Bolt, -1 Bloodstained Mire. I would also consider adding in 1 Copperline Gorge for one of the fetches or a stomping ground

Green was a good call. Atarka's command is just redonkulous in burn decks that run swiftspears and goblin guides.

I think burn without Lightning Helix is a mistake. It helps you race really well, because although burn is fast, sometimes a little lifegain to buy you one more turn for more burn spells can be really helpful. There are some decks that will kill you before you run out of cards in hand. In the mirror it's priceless, but in general it is super useful. Remember that every turn that goes buy equals about 2 points of burn damage, so if they gain some life early putting them out of an opening 7 burn kill, using helix to get an extra turn and 2 more points of damage can be super helpful.

Bloodstained Mire should be swapped out for mountains. Theres no black in this deck