Hiya,
Plenty of points for you to digest here...
1. Dont believe everything you read on the internet. Sometimes its wrong. :D
2. More importantly, this particular question is an issue of percentages. You start with 50/50 health. I can see from the mortem you posted that you ended on 0/50 health. 30 damage is 60% health.
3. Armour isnt very effective against plasma attacks. Red armour will only give -2 damage per shot, which reduces the arachs attack to 6-18hp. Thats between 12% and 36% health lost per burst.
4. Arachnotrons have a speed of 130%, meaning if youre not doing something very, very quick yourself, they will sometimes get 2 turns to attack you. In red armour, these 2 bursts can do up to 72% health. With no armour, they can kill you.
5. Reloading a double-barrelled shotgun is an incredibly slow action to perform. Arachs will probably get 3-4 turns in the time it takes you to reload that thing, especially since you didnt have the reloader feat. If youre going ammochain but want fun with shotguns until you get your plasmagun, Id suggest juggler, and instant-swapping between 4 different shotguns rather than wasting time reloading them in combat.
6. Youve already posted a mortem of an easy difficulty win using an ammochain build. I assume youre going for 100% kills here, but just wanted to advise you to try some different builds. If youd played a few shotgun games before, this character would probably not have died - variation makes you a better player, because you learn how to cope with a wider array of situations.
7. Running mode is spectacular against arachnotrons. Give it a try next time youre in a pinch.
8. Finally, you should have stood here:
=..........=
..#########.
..#|.|.|%|#.
..#s|...|.#.
..####.####.
.....%.A....
@.####A####.
..#c.....|#.
..#|\\\"|\\\"|\\\"|#.
..#########.
From that position, you can shoot the arachnotrons to death without them shooting back. If youre waiting for them to arrive, use the wait on the spot command (,.) until they come to you.
Hope those help.