From my perspective (And I'm no expert)
You could see 5 black mamba's and a Naga ritualist. Thats a lot of monsters. Use Javelins or stones to attract the attention of monsters in smaller groups from the edge of your LOS. Once it's noticed you - retreat back towards an upward staircase 5-8 tiles at least, and ideally around a corner. This ensures you don't pull more aggro than you can handle - monsters will often shout when they see you to alert other monsters in the area. If you move away from that area and out of view, you won't have to deal with the reinforcements they called.
Anything past that is just general advice to be honest. 5 black mambas is a lot to deal with, and it looks like you just finished berserking so were negatively buffed anyway. Don't berserk unless you are sure you can kill everything before it ends.
You did however have 5 potions of curing that could have stopped the poison. You also had scrolls of blinking. You may have been able to blink a few squares away...maybe around a corner. Closer to a staircase. Quaff curing and try and get off the level (Although black mambas are faster than you so probably would have caught you eventually)
You also had 4 scrolls of fear. Reading one of these could have caused all monsters to become scared of you and run away. Again, quaff curing, and run for the stairs. Or read a scroll of teleportation and hope you don't end up somewhere worse (Although I have read teleportation once in a panic and ended up on the square next to the one i read the scroll on!)
Finally you had potions of haste. If it was me, I probably would have read scroll of fear, if it worked, quaff curing, haste(if you were more than 6-7 squares from the staircase, although I'm not sure how/if haste even works after berserk), and head for the stairs. If it didn't work (Some monsters resist) then read blink to get as far away as you can, quaff curing, then haste if needed (As above) and retreat back up.
Aggro management is a massive part of crawl. I got my first win with a MiBe and its very easy to get complacent with just fighting everything all at the same time because they are very strong. But every now and then that tactic fails (As you are noticing!) I think at XL16 you were ok attempting snake:2 - you just made other mistakes.
Learning to manage your aggro and where and when to fight will massively help you with all races. I don't think theres another combo in the game you can be quite so gung ho with as you can an MiBe, so it'd be a good habit to practice being a bit more careful. Happy to jump on a server with you and show you what I mean with a more frail race