1. mostly viral + radiation + puncture against grineer & void & infested. The shields in void i never cared for as in higher lvls the warframes do the CC so weapons do the cleanup. Copus to magnetic + toxin.
Strangely i've had better results against infested with viral + rad. as mostly rad proc's them to dummies and base weapon dmg takes care of the rest.
2. there's a generic loadout to make every weapon viable (pure dmg mods + elements + personal preference) I tend to maximise the benefits on each weapon, boltor p you give away dps when modding for status, raw dmg mows down pretty much everything. A Tonbo needs high crit, it isnt viable otherwhise.
So if a weapon has high puncture I wouldnt bring it up against corpus (kills must be done quick), if a weapon has low status it will be a waste to slap on status mods.
For that i like http://dpsframe.com/
it is min-maxing (which i dont really like), but if you are up against high-scaling mobs it becomes necessary.
3. For me the minimum to mod into a specific attribute it needs 15% and more. If it has less I usually go for pure dmg. So the result should be at least 40% without sacrificing too much dmg.
For funbuilds on the other hands, a penta with radiation + high status + firestorm has awesome cc. or an Amprex with all out firerate is fun as long as you bring ammunition pads.
staves & poles with primed reach are awesome, even through walls
4. best one so far was venka with primed reach, so many phantom kills. Or multishot castanas with Mirage are great for crashing the game.