[Taqwus]

Know thy enemy -- be familiar with a nation's likely abilities, read their Pretender's titles, check the mercenary listings, perhaps use stealth to observe their battles.

Storms, darkness, fliers, unexpected mercenary heavy cavalry, tramplers, mages with area-effect spells, and so forth can really ruin your day if you're not prepared. Storms in particular shuts down most fliers and hampers missiles and fire magic, while darkness hampers most of the living in both melee and ranged combat. Of course, if you're casting the storms and you have fliers that will be grounded, you can have the fliers strike first while your storms caster waits.

Use misdirection. Some effects tend to target units with high HP, and there's also a 'target large monsters' option, so if your Pretender is on the scene, you may want to present other high-HP targets (elephants, some constructs, et al) to attract curses and horror marks. There's a 'target archers' command, as well as 'target fliers', so one can place sacrificial groups...

...and never, ever lead a horde of mindless units (ex. low-end undead) with just a handful of commanders with minds, against an illithid army, because the mindblasts will all target the commanders -- and if all commanders are dead, the mindless units will vaporize.

Be aware of how friendly mages might hurt you -- casting 'Touch of Madness' on your missile troops, casting 'Protection' on your Abysians, and so forth. Position and script accordingly. In some cases, it may be safest not to bring along a mage with an particularly dangerous path.

Be wary of relying too much on awe, fear, tramplers, and etherealness. While these can be powerful... awe and fear are morale effects, and there are many mindless units that will ignore them; tramplers have a hard time trampling ethereal units; and ethereals should recall that there are even national troops with magical weapons.

With Communion, self-effects also affect the communion slaves; this includes such things as Summon Earthpower's reinvigoration, Invulnerability, and Body Ethereal. The first makes your communion even more efficient in dealing with fatigue (reinvig 4 x number of communicants, rather than just reinvig 4), while others make them harder to kill. Even Phoenix Pyre carries over, but this is... dangerous, unless your army is fire-immune.

Ritual magic, including remote summons and any resulting battles, take place before all non-magical movements and attack; this allows one to catch enemy forces and disrupt them, while riddling their commanders with Seeking Arrows. There's a reason why Ghost Riders is Conj-9.