For angel of light travel, a pistol game is a pretty good choice -- that was the first way I did it. If you do this, I would still try to get a bfg. Its also possible to go chaingun in angel of light travel. The added damage is nice on harder difficulty levels, but it may be a good idea to not clear out every level in an effort to conserve ammo. Totally switching to plasma is probably impractical. I'd say plasma has the worst power per inventory space.
What's the best trait path for a TaN/Badass/Ironman build?
I'd say you don't want to go pure TaN/Badass/Ironman. As a chaingun/plasma specialist, I'd try to get at least 1 level of eagle-eye and at least 2 levels of SoB. In general, I'd say TaN is more useful in the early game and Ironman is more useful in the late game, but in AoI I'd lean towards Ironman even in the early game because you generally have enough inventory space to lug around quite a bit of extra armor. I'm not ordinarily much of a Badass fan. I would suggest something like this:
EE->SoB->Iro->SoB->Iro->Iro->SoB->TaN->TaN
Keep in mind that trait orders tend to be quite flexible (e.g. you'll get to level 2 on level 2 of the phobos base, so the order of your first two traits doesn't matter after that point).
Also in AoI, I wouldn't try to clear out every level towards the end, especially if you already have a thermonuclear bomb. Though its harder in AoI because of lack of homing phase devices, it is also more important. Being able to carry health packs helps smooth out the availability of health recovery, but in AoI you are completely at the mercy of the RNG. If a level happens to spawn with very little or no health, you will be in big trouble. So, once you have found the stairs, explore as much of the level as you can for health without engaging enemies, then go down, unless there is enough left-over health (that you have already found) that you think clearing the level will be relatively safe.
