Peace talks. Divide the land in half for each country, then follow through with the next plan.
Were it that we had the option to let them fight it out, I would use guerilla warfare agains tthe 'superior force', along with Ghengis Khan's horseshoe tactics, and Sun Tzu's subterfuge, i.e. fake intel and wars of attrition, combined.
Hit the edges of their army with small raiding forces, use my entire nation as a battlefield to wrap around the army and cut off their supply lines, and then in actual skirmishes and battles, I'd march at them in straight lines and columns, using the phalanx from the Greek times to keep the soldiers hit alive, while collapsing the middle of my columns to fool the enemy into thinking they had broke ranks.
Once they'd been lured in, turn around and crush them with a flanking from all sides, plus two or three other columns on each side of what was left of the enemy's main army. After that, I'd use small, roaming paries to mop up any resistance left.
Tactical genocide, brother. :3
My scenario -
You're Athens, and Sparta is attacking. Your country is riddled with plague, so you have inferior weapons and armor, your horses are dying, your food is bad, and your soldiers' numbers have been cut in half, not to mention they're weak from lack of food, and your soldiers were less well trained than Sparta's in the first place.
You have but two things to your advantage.
Your navy is still at sea, and thus unaffected by the plague, and your city is one of the best fortified of all the Greek city-states.
These are your options...
In regards to your navy, you can do four things.
-Use them to ferry supplies and medicines to your city from other Greek nations, and run the risk of infecting them too.
-Have them help in the defense of the city, guaranteeing infection, and putting an even further drain on your dwindling resources.
-Order them to assault the Spartan polis
-Have them attempt to hire mercenries from other nations
In the defense of your city you can do one of these things...
-Melt down horseshoes, daggers, knives, anything that's metal and not being used, to make more weapons, and cannon balls
-Make armor and arrows out of said metal
-Make a few catapaults to fling the corpses of your disease ridden people into the opposing army.
Remember, this is only to save your city from being sacked, not annihlate the Spartan threat.