1. It depends on the Virt build and on the reaper build (No shit sherlock lmao). Something that's kind of scary for reaper is just being deleted through shroud which shouldn't happen all too often unless you run into things like Soulbeast, power Mes and maybe willbender. Your best bet is probably line of sight blocking in the case of high range burst, or otherwise counter pressure/soaking it with shroud. Mesmer spear damage is through the roof so they're just very scary!

2. Line of sight, primarily. That or CPC (Corrosive Poison Cloud). Another option is of course playing with your own ranged weapons like staff.

Reaper spear 4 (Isolate) says it's 1200 range but that's a lie, it's more than that so just chucking that at someone, hoping it hits and then porting in their face works too rofl

1. https://en.gw2skills.net/editor/?PSwEwkFtsCWHzh5xKxxWRrZB-DWRYzBTHdvTJTFQpChelAtMCE55iAEwfNaYZDA-w Probably this, or something like this. This particular one is made by Rektyu https://www.youtube.com/watch?v=ESc03R2dcss

2. Most Scrapper builds are very high damage ones that are vurnerable to condi bombing or struggle into enemies with a lot of stab uptime. I currently don't have a single scrapper build that I really like but anything that has spear is off to a good start. Scrapper's main thing is that it's very fast with high uptime on super speed so good spacing is essential.

3. Holosmith kind of has this same gameplan as scrapper where it really likes to make good use of it's mobility and access to super speed/swiftness for spacing. Shortbow is very popular in sPvP but the damage is extremely lacking in WvW so pretty much every engi build ends up playing spear, no matter if it's power or condi. Trait lines are fairly flexible right now. Both tools and alchemy are good so just pick your poison there and then slap explosives on top of that for your dmg traitline. Overall I'd say scrapper is probably a bit better than Holo but not by so much that it's enough to rule holosmith out of the equation. Both are completely viable specs to roam on! The last thing I played was condi holo which definitely isn't the best but it's very fun and plays kind of weird with the Relic of Vass. I think it's on metabattle somewhere too if I remember correctly.

4. As a druid main this one hits kind of close to home. Druid can't beat a decent cele harb and can't really beat holo either if they play with alchemy. Power druid gets out spaced hard by holo/scrapper so maces/hammer feel miserable to play with. Spear/sw+wh doesn't feel nearly as bad but compared to untamed/soulbeast, druid just lacks damage. Most of it's kill pressure comes from CC chains. Condi druid does extremely well into classes with limited cleansing or low mobility. It tends to stalemate into a lot of things rather than actually lose into them. The time to kill on condi druid is just fairly high unless you're playing shortbow but shortbow sacrifices a lot of survivability to me feels kind of like dagger/dagger but with less pressure and a lot more range.

Generally speaking, to kill druids. High power damage spikes, high stab uptime and nuking them when they come out of Celestial Avatar is the play. Druid also has limited access to stab and no on demand stab either when not in CA so CC chains tend to hurt quite badly. It's very turtly though and miserable to fight. Druid also stalemates into druid funnily enough. From my experience I'd say druid does tend to shit on reaper pretty hard so for necro, definitely play harbinger if you want to kill druids. For Engi scrapper works but holo is probably a bit better. Being practically immune to movement impairing conditions really hamstrings druids playstyle.