so...Imma try to summerize the argument here
(1): the city walls,buildings stuff, etc, would give the nid's room to pull harrasment...(side A)
(2): fields are good for harrasment (side B).
side B's premise however is only true sometimes, if you are trying to advance /attack them aggresively, then highly mobile harrassment benifits from barriers as they can use them to skirt the enemy and prevent solid sightlines. if you are defending however, (which we most certainly will be as nids invade, they don't defend), then obstacles only act as funneling for the enemy.... which makes harrassment near-impossable.
but thats assuming symmetric warfare(same types and similar numbers) which we know this won't be the case....but we can't easily make that determination of what type of battleground would benifit us most til we actually see it in action. (yes, we know they are using psykers, steath and mobility, but we need more details then that...IE, do they use biomancy backed by divination? or maybe they use pyromancy? (unlikely I imagine)....)
however, we can guess that IN GENERAL, the city walls will generally benefit us more then the nid's under most circumstances as we are defending, it has been a time-honered tradition and time-proven that walls almost ALWAYS benefit the defenders to some degree regardless of circumstances...because even if the enemy has some way around/threw them, the walls still force the enemy to USE said tools and waste time and energy.
.....so we would still want to force them to land outside the citys (if possable) and then travel to our citys and meanwhile be bombarded by artiliary then attempting to get past the walls while being engaged by our defenses and only THEN fighting us in melee.....
course the nid's might find a way to skip one of those steps via psykic powers (or just landing directly in the citys...can they do that?)...but why let them NOT skip a step and thus use up some of their resources?
honestly, we cant do a whole lot of pre-battle optimization till we know more details of how they will be fighting.
.....
SO, time for guess-work! cas we can't do anything else.......
I think its almost explictly given by rid's divination work that the nids will be using ultra fast and steathy melee-type and sniper-type units backed up by some equivalent of psykers which reinforce prior mentioned quality's with biomancy and other types of warp powers...this is less guess work and more of a certainty..
..however,...offensively speaking they might use:
(1): divination to gain info-advantage,
(2): metal-bending to increase chances that the cron's can't heal,(is that a thing? the nid's might make it a thing),
(3): time and/or space manipulation to control the battlefield in a attempt to ensure the enemy cant get good sight-lines on them while still retaining the ablity to strike themselves.
(4): I want to guess that warp-fire is as effective against the cron's as it is against anything else....so they might use suicide-bombers which just charge us so that the nid's can actively use their over-whielming psykic power to FORCE the resulting hole even larger.
(5): I don't know the kinds of confirmed types of pyskic powers in wh40k...some people should throw down a list so that we can guess which ones the nid's would try to use against the cron's in space. (although the nid's could very easily invent their own kinds....)
(6): some kind of warp-shinanagins to try to prevent cron solders warping out when defeated....the shadow in the warp might not be perfect afterall.
I suspect that some well-placed psykers that are powerful enough to resist the shadow-in-the-warp and still remain battle-effective and also experienced with anti-psyker fighting could easily change the tide of the battle....
*over hears whispers*...."what? oh, yeah I guess your right, they ALWAYS can change the tide......but even MORE so then usual I mean.
we might want to spend a THIRD div action to find details of how they fight...