Hey, I got your message regarding FAangband. I didn't know I was the only one to beat FA with a High Elf assassin. I'll have to verify that lol. To be honest I think that with most of these rogue likes you need to find a way to get your character to a point at which they can get a couple of reliable escapes and or more importantly find a game changer weapon.
With the High Elf assassin or any high elf for that matter I tend to stay close to the gates of Gondolin. If there are connecting stairs I would say stay close to the stairs and look for easy foes that give a lot of expo so that you can get a few levels right off the bat. Speaking of bats if I recall the lighting (blue) bats and the fire (red) bats are good for some quick leveling. I think assassins are good with ranged weapons (its been at least a year if not two since I played ) So basically my advice is get some easy levels and then start picking off the weaker orcs. Generally with some good ammo you can pick them off pretty well just have blink scrolls handy if they get close. I think the fact that FA isn't always in a dungeon environment might be throwing you as well since its harder to funnel foes into tunnels.
In general you want to start slow and then you can be bolder and start diving once you get some decent gear. But if you can beat Sil....FA should be no problem since like Angband you have a lot more opportunities to control your environment. In Sil this isn't the case. You have fewer opportunities to control your environment and I tend to go the guerrilla warfare route with Sil. But that is what I can think of right now if you have an more questions feel free to ask.