what useless skill should i max next?

(My thoughts on each skill are placed into individual spoilers to shorten the post length.)

Sacrifice

 Spoiler

Contrary to popular belief, Sacrifice is not good for Berserk activation while training. Its damage is inconsistent (and therefore its HP reduction). What Sacrifice ends up is aggravating the monster while doing variable damage.

Activating Berserk with Sacrifice with full HP: My Berserk HP is 4.9k (unwashed). My HP is currently full. I can Sacrifice a Skelosaurus a few times to get my HP down to Berserk level. However, in the process, the Skelosaurus has buffed itself (it buffs itself and surrounding monsters before attacking unless it has its own buff already) and has begun to beam at me. At this point I've done an inconsistent amount of damage to it that will allow me to 1~2hko it with Berserked Crusher.

Activating Berserk with Sacrfice with partial HP: Same scenario, but I now have 1/2 my full HP. A single Sacrifice brings me just under Berserk HP (yay), but it aggravates the Skelosaurus and it beams me. Now I still need to Crusher it 3 times, meaning I get beamed twice as opposed to the normal single beam. I would rather just run into the monster to lower my HP. What's worse is that Sacrifice is inconsistent, so it may bring down your HP too much, forcing you to pot.

Activating Berserk without Sacrifice: My Berserk HP is 4.9k (unwashed). My HP is currently full. I Crusher a Skelosaurus, it become aggravated, buffs itself, and begins to beam me. It beams me 2~3 times before it dies, bringing me down to or barely above Berserk HP (in which case I would simply touch another).

Though, I'm not saying Sacrifice is completely useless. It can be used to KB bosses, although Rush does the same thing at a greater consistency. It can be used as Berserk activation, although this, I've found to be only good for bosses that 1/1.

Sacrifice being an effective Berserk activator: I'm at a Left Pianus. It 1/1's me. I use a Ginseng Root to 40% HP, greater than my Berserk20 threshold. I sacrifice it once, and boom, I'm back in Berserk.

But that's my two cents.

Iron Body & Iron Will

 Spoiler

Iron Body's +40 W.Def is negligible when training, as it's damage reduction will be very little. It is also nullified each time you get Rage or Bless, making you rebuff it. It also is replaced by the Hex of the Beholder (80 W.Def and M.Def at Lv20; 100 at Lv25). The same applies for Iron Will's +20 W.Def and M.Def. I only use that skill because I only have to hit that key every 5 minutes. The only noticeable use for Iron Body and Iron Will is to reduce small amounts of damage (like 20) to 1.

Power Crash

 Spoiler

Power Crash... I've never actually found a use for it (and therefore left it at Level 1), although Stereo brings up a point about HT's tail. I suppose that's the one use we have for that skill at this point (even if Dispel = Power Crash + Armor Crash + Magic Crash + Party AllCure with a much larger range).

What I would do is put 1 into PA Crusher, 1 into PA Fury so you can play with PoleArms a bit more (since you can already Sacrifice, Roar, and Rush with them), 1 into Power Crash for the animation, and allocate the other 14 SP into whatever you feel like you'll use most. If anything, you can use the 17 SP to max Iron Will to buff your friends.

As for 4th job Skill Build, there are a lot of factors that go into it like your HP, if you party or solo, what equips and base DEX you have, and whether or not you have access to Berserk 30. The skills you want to max (or get as close to max as possible) are [in no specific order] Beholder (the summon), Berserk, Power Stance, and Maple Warrior, with 1 point in Rush. The rest aren't [as] useful and can wait until after.

Apologies for the overly-extended post. Hope it helps in some way.

Edit: Spear Fury is not useless. Spear Fury acts as a 4th Crusher hit (it does a tad less than a Crusher hit would), but is faster and has greater range. It's also useful as a luring skill (for faster mobs like Dreamy Ghosts) and a weak mob killer with its speed and range (good for quests that require you to kill and/or collect some drop from weak monsters).