Does spatial psychosis proc the check for falling if an enemy is prone within half an inch from a ledge?
As per the Rules if a fighter directly causes another to go Out of Action, yes they get the XP. If the Psyker didn't cast the Trip and Fall power the enemy would not have fallen into the acit pit filled with Laser Scorpion Sharks (It's Necromunda after all) so it is directly causing the OOA. If a Goliath Stimmer picked up and Hurled a opponent off the edge and they fell to their death they would get the XP. Its the same thing but with creey fish mental powers instad of 'Roid Rage muscles.
If not what alternate options does he have for XP gain? Outside of just punching people to death that somebody softened up for him?
As Per
@HugeSamples they earn XP by Turning up, Rallying, Assisting another Recovery, Scenario specific rewards and taking Fighters and vehicles out.
You only need a singe 3XP upgrade to get Willpower 7+ which is ok for powers (If you use the Concentrate (Basic) action that adds +1 to the roll)
"punching people to death that somebody softened up"
That is actually really Viable for a Psy-Gheist in a Delaque gang. For the Creepy Bald Assassins there is a loverly French Phrase. Coup de Grace. Perform a Coup de Grace on a Seriously Injured foe and you take them OOA regardless of wounds. Or Armour Saves etc.
You can have Web Guns and Pistols spinnereting out of every single fighter (The Naght-Ghul can Trade Post it). Both weapons are Templates so can catch multiple foes. With either S4 or 5 (thats good to great), and no Armour save (that's Awesome). They bypass the Injury dice and go straight to Webbed instead (So no Flesh wounds and cralling away that turn).
While webbed they count as Seriously Injured in almost all respects (no additional Flesh wounds if they don't recover in the End Phase which won't matter)
As they count as Seriously Injured they are vunerable to the Coup de Grace action.
One Web Gun Shot and a sucessful Web on a Leader or Champion (most of which are T3 or 4 so quite easy to Web) and a single Charge by a Psy-Gheist and they net 3XP. Ignoring all Armour. And they can do it again next turn. Even if you do it to only one Gang Fighter a game that plus your bonus for turning up grants you 3XP (or that Will power upgrade) every game and with Fast Learner it remains at that 3XP cost. In one Game I have earned 9 XP from that tactic and four poor Bone Pickers with one Web Gun Shot (poor beggers couldn't roll a Flesh Wound result in the End Phase to save thier life) . 8+ Willpower to 5+ in one Game.
It gets even easier with the right equipment. The pricey 110 Credit Psychomancers Harness. That bumps your Psy-Gheist up to Move 8", that doesn't Half climbing distance so can keep up with Dirt Bikes and pounce over many gantries hunting the victims down with ease. And the Claws give a 3" Versatile range Thats a near unfettered 12-14" Charge range for Coup de graces. Should you Charge a none Seriously Injured Foe (assuming you haven't upgraded you Phy-Gheists physical Stats yet) you get Three attacks At S4. That Ignore Saves. And if you roll a natural 6 that attack Auto Wounds and with 3 attacks there is a decent chance that will happen.
Charges like that seriously mess up any Gang Fighter even if they are not softened up by others. Add another Attack upgrade and thats 5 attacks on the charge. That can instant OOA many champions, leaders and Brutes with some moderate luck and spells near certian OOA to any Gang Fighter.
With that Harness you don't even need the wierdos mental powers. You can just use them to hoover up Gang fighters and rake up kills. Team them up with Webbers and they can soon outpace your other Gang Fighters and boost themselves into champion threat levels. Without Casting a single power. If you go Combat monster the Powers are just icing.
Normally I would not bother Promoting Prospects and just utilise the Fast Learner rules but a Harness Psy-Gheist I would make exceptions for. They can easily rack up enough advancements (5+) to qualify for Promotion in Down Time. Pump the one Allowed into Attacks (2A) and Strength (S4) another into WS (WS3+) and another into Toughness or the single Wounds you are allowed. (lets say Both T and W to make 5) You now have a M8" WS3+ S4 T4 W2 A2 fighter
Thats 46xp which looks a lot lets assume 4 games in the first phase -4 for turning up so 42 XP so 10/11 per game. Which is 4 bone pickers (juves) and a champion a game. This is possible over 4 games I have done it twice (Emperor bless Cawdor and thier hordes of expendable Meat Shields).
OK you could not go for the Attack and Wound and pick 2 Will instead (24 after turning up XP which is is only 3 juves/ganger kills per game without having to hunt champions)
Then Promote them to Naght-Ghul. That Movement Plus the Phased Shock Versatile claws and From the Shadows is brutal. Plus your Maximum Characteristics is limited to "the basic profile for a fighter of thier current catagory". Which is now Naght Ghul not Psy-Gheist. So you could boost Attacks (and Wounds) up to 3 and S5 coupled with the Paired claws. A 7A S6 Phased Shock Versatile Shock charge is a recipe for Opponent Paste.
You can really rack up the combat power and XP with a cunny Psy-Gheist and a little web cooperation and ruthlessness with opponents Gang Fighters.
And thats leaving your Master of Shadows and Chanpions free to take down real targets.