I'll assume that the amazons are greatly outnumbered. However, I don't see from the question how being an all-female force actually matters. Unless we go into psychological stuff, how man hardwired to protect woman and so on. Or state that they are physically weaker, smaller, etc.
Some parts of my answer are invalidated by the edits in the question. I'll still leave them in, only ignore/skip them.
So, my answer will be weaker, defending force against stronger, attacking one.
Asymmetrical warfare. Since its their home turf, fortifications to prevent or at least inflict maximum damage during the landing. Build the fortifications to be easily abandoned and stealthily booby trapped, if the invaders want to occupy them.
Extensive tunnel systems, like the Vietnamese did. It provides undetectable movements for rapid attacks and retreats, or overall troop movements.
Guerrilla tactics. Don't attack their strength, attack their weaknesses. Supply lines, logistic, night raid on their camps from safe distance to wore them down. Destroying is easier than building. Torch their ships, if its an island, drill holes into them if they are exceptional divers.
Best examples, today's terrorists vs USA army.
After additional thoughts:
(kudos to LSerni for bio warfare idea - not sure how to link his answer here)
Level 1: Mountains with extensive cave systems and thick jungles.
Level 2: Numerous, small, fast rivers which make advancement and troop concentration hard. The shallow parts are rare, tricky, making good ambush points. Alternative: deep valleys and narrow canyons.
Level 3: Hard to sail around the island. Reefs, whirlpools, storms, fog. Bermuda-Triangle.
Level 4: Harsh, changing temperatures. During day, scorching hot, high humidity, during night, freezing cold (don't go together with jungles though) The islanders adapted to these conditions. Thought its yet again, not about females.
Level 5: Bio warfare - Highly poisonous insects, plants, allergenic pollen in the air. The islanders either became immune/resistant over the years, or have the cure. This can be scaled, depending on the desired impossibility of the invasion.
Hell, even just after level 1, I would most likely decline to lead troops unless really want to squander them and have 1 to 1000 ratio with unlimited logistic supply. I would simply torch the hole place if possible and not even that would work for sure. Maybe siege, but from description they are plenty self-sufficient.
Thought it's not about the terrain, but if the amazons are well disciplined, have no easily distinguishable features from the Empire's populace and want to win:
Deploy squads who infiltrate the home country and sabotage them from within. No need for terror incidents, that's just antagonize the population. Information gathering, propaganda (poor, helpless woman needs help and will reward all assistance very warmly), assassination of key personnel (mostly those supporting the invasion), sabotage of supplies, turning their soldiers against them.