[RPG] What are all the ways that a PC can become immune to the frightened condition

Phantasmal Killer has been errata'd http://media.wizards.com/2015/downloads/dnd/PH_Errata_1.1.pdf

Phantasmal Killer (p. 265). The frightened target makes a save at the end of its turns, not the start

So the complete exact text is:

You tap into the nightmares of a creature you can see within range and create an illusory manifestation of its deepest fears, visible only to that creature. The target must make a Wisdom saving throw. On a failed save, the target becomes frightened for the duration. At the start end of each of the target’s turns before the spell ends, the target must succeed on a Wisdom saving throw or take 4d10 psychic damage. On a successful save, the spell ends.

IMHO DM Ruling is required here.
That successful save refers to any save for that spell or just one of the "avoid the damage" saves?

Since the errata says "the frightened target makes a save" I'd also rule that if the target has the frightened condition removed the entire spell ends.

Please note that this strict ruling makes this 4th level spell sub-part.
Feel free to give Phantasmal Killer some DM love :D

Edit:
After more general considerations and a lenghty discussion with friends I feel the need add this:

Lets consider Vicious Mockery

You unleash a string of insults laced with subtle enchantments at a creature you can see within range. If the target can hear you (though it need not understand you), it must succeed on a Wisdom saving throw or take 1d4 psychic damage and have disadvantage on the next attack roll it makes before the end of its next turn.

If cast on a creature immune to psychic damage should I roll the save? Yes, because a failed saving throw determines two outcomes, psychic damage and disadvantage on the next attack roll. If the target fails the save it would take no damage but still suffer the disadvantage.

So, back to Phantasmal Killer:
"On a successful save, the spell ends." seems pretty clear to me. If the save agains fear succeeds the spell ends, if it fails the spell goes on even if the target is immune to the frightened condition. This is brutally RAW.

I very much doubt this is RAI, especially considereing the wording in the errata. In any case, forgoing the first save its a bit too much.

The short answer:

AC 0-5: Human Great Weapon Fighter with the Great Weapon Master feat wielding a Greatsword

AC 6-20: Raging Human Barbarian with the Polearm Master feat wielding a Polearm

AC 21-26: Half-Orc Two-Weapon Fighter

AC 27+: Human Great Weapon Fighter with the Great Weapon Master feat wielding a Greatsword, part II: the GWF returns.

The significantly longer answer is that here are some calculations I did for the options that seemed obvious to me. Interestingly enough, the Barbarian didn't make the top of the list in terms of pure damage, but it did make the top of the list once we added AC into the mix.

You can also add Caltrops to any of these builds (and you can afford a lot of caltrops) to add up to 1 dmg/r.

Other notes:

• The only source material used for this was the PHB, as I do not have access to all of the adventure paths and supplements
• Magic users need not apply: Cantrips are almost universally low-damage, and they never get to benefit from base stats to get those sweet, sweet flat bonus damage numbers. All other spells are too consumable to be considered, so they didn't make it to the number crunching stage.
• Rogues were briefly considered, but sneak attack just isn't reliable enough when you can't count on nearby allies or advantage.
• No cheese! I tried to stick to a purist approach here. There may be strategies that your GM approves like this but most reasonably GMs would ban that kind of build from their tables for being too OP. This list fits nicely within very vanilla RAW and RAI.
• I gave all characters rolls of 18 for their stats, then added any racial strength bonus that applied. Point-buy only allows you to get to 16/17 instead of 19/20, and will lower your dmg/r across the board.
• All damage is given in expected values, done by hand. It's possible I failed my Intelligence(Statistics) check, so if you see something say something
• Humans used are Variant Humans, to take advantage of the bonus feat. Half-Orc is used otherwise as they're the only race I could find that can possibly add damage to their attacks through the Savagery feature, as well as giving a handy +2 Str.
• No magic weapons are used, as they are out of the budget of level 1 characters even if they sell all of their possessions, max out their starting gold rolls, and take the Noble background
• Using the point-buy system the half-orc takes a bigger hit than humans to both his to-hit values and his damage (the +2 doesn't get him to a higher Ability Modifier tier). This results in the Barbarian Polearm Master reigning supreme all the way to 24 AC, and the GWF picks up the slack at 25+, cutting the half-orc out of our equation completely.
• You want numbers? Here's a Google Sheet with some numbers.

Without further ado, here is my list of damage-optimized level 1 characters (please refer to the above sheet for numbers with AC factored in).

Human Great Weapon Fighter with Great Weapon Master feat (22.75 dmg/r, +1 to hit)

• +4 str bonus
• +10 feat bonus (-5 to hit)
• 2d6 (reroll 1,2) greatsword attack (22.333333 dmg/r)
• 5% crit (.41666666 dmg)
• Total damage on maximum Crit: 14+12+12 = 38 dmg

Human Barbarian with Polearm Master feat (20.3 dmg/r, +6 to hit)

• +4 str bonus
• +2 rag bonus
• 1d10 polearm attack (11.5 dmg)
• 1d4 bonus attack (8.5 dmg)
• 5% crit (0.4 dmg)
• Total damage on maximum Crit w/ both attacks: (6 + 10 + 10) + (6 + 4 + 4) = 38 dmg

Half-Orc Two Weapon Fighter (17.4 dmg/r, +7 to hit)

• +5 str bonus
• 1d6 shortsword attack (8.5 dmg)
• 1d6 shortsword attack (8.5 dmg)
• 5% crit including extra damage die from Savagery (0.7 dmg)
• Total damage on maximum Crit w/ both attacks: (5 + 6 + 6 + 6) + (5 + 6 + 6 + 6) = 46

Human Two Weapon Fighter with Dual Wielder feat (17.2 dmg/r, +6 to hit)

• +4 str bonus
• 1d8 longsword attack (8.5 dmg)
• 1d8 longsword attack (8.5 dmg)
• 5% crit (.45 dmg)
• Total damage on maximum Crit w/ both attacks: (4 + 8 + 8) + (4 + 8 + 8) = 40 Dmg