Is the "One Steppe at a Time V" achievement reward a bit cruel to put up?

[Cidel]

I've even taken up to trying out different classes, the gameplan seems to be Tanks being able to become virtually unkillable for a time in a timed event so I've gotten to practice being an utterly toxic unkillable bean for bursts of 10-30 seconds,

Well yes, that is the gameplan depending on which tank you're playing and the general IQ of your team/enemy teams. The only 2 tanks that come across as "virtually unkillable" would be DRK and PLD due to the nature of their LBs. In a FL environment, PLD is far more effective at being a distraction/nuisance with a side of clutch covers/stuns for finishing-off people. DRK is... a meme at this point so I think it goes without saying what it's capable of.

WAR is more of a disrupter/ set-up role. And GNB is probably the most glass-cannony of the 4 that requires an aggressive team to do well in. Not to say they aren't "tanky" either, they just lack the emergency "how to keep morons occupied for 10 seconds" buttons DRK and PLD have.

...Untill the entire team focuses you with more stuns on various classes than what seemed feasible.

That's a positioning issue. If you're ever caught in that situation you likely overextended or overstayed your welcome/ your team left you. Up against upwards of 20+ people, you are bound to get chain-CC'd.

...Still can't figure out how BLM nukes so many people without being killed for it constantly.

Any BLM worth their salt in FL is keeping a safe distance or surrounding themselves by other teammates. What works in their favor is that your average FL player is unable to look (or tab) beyond the DRK or DRG in their face to the healers or ranged DPS standing 3 feet behind them with a significantly smaller defense bonus modifier. People often don't realize that a BLM will take a fraction of the effort/damage required to kill than any melee job. I've seen people ignore ranged DPS in situations of a choice between them and a melee/tank just because the latter wasn't quite at full HP...

...Sage being able to hold areas has felt utterly useless since everyone seems to know how to drop an AOE inside the shield making it a useless killbox.

Sage's LB in FL is a perfect example of the This Looks like a Job for Aquaman trope. It's useful in bottleneck areas like Borderlands' caves ramps and narrow chokepoints to the middle of the map, or bridges and mid ramps in Onsal. If you don't think you can get a use out of it defensively, consider dropping it in an enemy chokepoint the same way a MCH drops its turret as a deterrent. Their LB in Frontlines is rather underwhelming in most scenarios and you're better off focusing on swapping your Kardia target around to yourself and your party members most likely to be hit so you can buff your Toxicon's potency after applying your DoT and applying your defense-down AoE as often as possible to help things get killed.

I want to continue using Machinist but kills seem to be only possible for groups of people overwhelming someone with their multitude of defensive/healing abilities and I too often found myself focusing the wrong person. (sniping is still fun though)

MCH can be tough in FL, as they're initially quite underwhelming against tanks and melee DPS due to their highest-level defense bonus modifiers. You're far better off picking-on other ranged and healers. And without high teammate support, your damage has to be fast and concentrated. Otherwise your target will just outrange you and heal-up everything you just did. MCH's turret is great space-control; most people will immediately clear out of wherever you drop it, otherwise eat chip damage and a defense-down debuff. MCH's LB seems to be the hardest thing for people to use effectively. I often see people wasting it on people near-full HP/ People in their Guard bubble (it doesn't penetrate Guard, only buffed Drill does that)/ or on PLDs and DRKs in the middle of their LBs. Unless you are rocking a high Battle High level, their LB should be used on ranged/healers sitting at less than 60% HP; preferably after a buffed Drill or Air Anchor.

The focusing the wrong target is something a lot of people seem to struggle with. As a general rule, unless you absolutely have no better options, higher priority targets will always be healers and ranged DPS. Until you hit Battle High 3+, you're basically tickling melee and tanks without concentrated team fire. It's unfortunate, but I'm sure a lot of people aren't even aware that in FL, tanks and melee have a large defense bonus compared to healers and ranged. Such information isn't exactly made apparent for some reason.

...Or swearing that a Paladin somehow effing Holmgang'd you, Paladin's don't have a yank move right!? wtf...

They don't. They have a potent close range, single-target stun on a 15 second cooldown and a 5-second AoE Heavy if their Sheltron barrier isn't broken before it expires. You probably got drawn-in by a WAR or DRK in close proximity of said PLD.