@Sian
Actually, it depends on several factors, the most reliable one being your spells or early holy site. If you have invisibility, any ranged unit can do it (even the 1st tier). At the current state of the game, it is useless to concentrate your army to quickly conquer other planes, imo you better use few units buffed with invisibility for example to do the task, then use the bulk of your army to conquer another AI, unless you want to play around. The only resources worth in the plane in current state of the game is the lootable sites, adamantium and nevril and dragons are worth if you can rush it.
add:
Another unorthodox strategy to conquer plane quickly or quick rush gold dragon/nevril/adamantium (Note: This depends on luck) : Have a ranged unit, 1 to 3 units, depends on their attacking power, they should be able to counter the Gold Dragon regeneration plus healing from resting, buff all your ranged units with invisibility, quickly look for Gold Dragon and ignore everything else except some lootable to boost your gold if you don't have enough gold to buy a gold dragon, take it down, build a city as soon as possible there (you can cast invisible on your settler to tail/follow the invisible ranged unit party then after the gold dragon is dead, you can immediately build a city there, if you have teleport, you don't need to do this), then put your invisible ranged unit in the city, this will reduce the alert of most monsters in that plane, meaning your city won't really bother them like if you put a non invisible unit there, now your city is waiting to build the first Gold dragon. You can use this 1st gold dragon to wipe out the plane or AI or quick rush adamantium and nevril, can quickly wipe all AI if you teleport it near their capital and ninja snipe their capital (not my style though). If you are lucky, meaning you discover a portal to another world and invisible spell around turn 40, you can have your first gold dragon around turn 65-75, then if you concentrate on getting all lootable, you might have the 2nd goldie 5 turn after the 1st one is created. Flyer is the most useful in island setting map.