Frogs have so many disadvantages as a weapon system. They are tiny, fragile and retiring thus making them useful only for indirect, sneaky, nasty attacks. Frogs can't even reliably climb stairs. Poison is the only equalizer here since direct physical trauma isn't possible with this weapons delivery system.
Whatever chemical weapon the frogs deliver must either kill the minion outright or cause him to kill himself or attack another minion and kill them. Personally, getting the minion to kill himself or one of his friends is the most satisfying, so let's go with hallucinogens. Anyone who's unprepared for a hallucinogenic high is really going to have a hard time. Their superstitious minds will come up with angels, demons, devils...whatever their world-paradigm offers.
Let's assume that there's a frog that produces such a hallucinogen, one that causes visual and auditory hallucinations but also triggers the brains various threat centers. The effects are a mashup between Google's Deep Dream and Scarecrow's Fear Spray. Let's give it a nice long half-life of 18 hours before the effects wear off.
The Latest in Frog Gear
Equip each frog with a time delayed sack attached to their back. Let them lose in the barracks, kitchens and in higher places in the fortress. After the time delay kicks off, a tiny hole appears in the sack and the weaponized hallucinogen begins to spread whenever/wherever the frogs hope. If the powder is fine enough, it may be an inhalant or a contact poison (either way, it doesn't matter. The minions won't have any idea what hit them.)
In a matter of hours, the fortress will tear itself apart as everyone exposed to the hallucinogen temporarily or permanently loses their mind(s). Heck the strain may be so bad they develop second minds and lose those too. Even if the overlord survives (unlikely given how he's an object of fear among the minions), he'll never be able to raise an army again given the terrifying way he lost this one.
Poison the water supply
Standard survival knowledge says that if you see a dead thing in your water supply, don't drink from that water supply. Dead things equals some really really funky stuff. Further, the bite of the monitor lizard is not in itself lethal, rather the infection that comes from that bite causes massive sepsis and then death. Contaminating the water supply will lead to sickness or death for a significant portion of the fortress.
How you gonna do it?
Collect a few hundred frogs, (doesn't matter what kind) then kill them. (Exactly how many frogs you need will depend on the size of the stream feeding the fortress. Bigger streams will require more frogs. Sorry frogs!) Mangle the bodies a little to increase the surface area. Put that all in a big barrel and toss in some manure and raw pork for good measure. (Raw pork in anaerobic atmosphere may get you the holy grail of poisons, botulinum toxin. It's worth a shot.) Seal the barrel, leave it in the sunshine for a few days so it's nice and ripe. Go up stream of the fortress and take the barrel with you. Transfer the contents of the barrel to several burlap sacks. Build a small dam to make a quiet portion in the stream. Tie each sack to a large rock and heave it into a dammed pond. The juicy mixture in each sack will leak out and contaminate the water. Repeat this procedure several times till the fortress is a puking their guts out or dying.
You could have used any animal for that
Perhaps but as the OP states, there are lots of frogs around. Frogs aren't especially useful as weapons, outside the application of poison frog toxins to arrows or swords. Frogs can't be herded, they have a long growth cycle that must occur in an environment that humans don't especially like and they die really easily.
Disguise yourself as a holy man of some kind. Lower tech levels generally means an increase in superstition so it shouldn't be hard to foist yourself of as a prophet. Start spreading rumors that frogs are a very evil omen and that anyone seen in the company of a frog will soon die. Kill a minion or two in a really gruesome fashion then sprinkle the area with enough frogs to garner attention when the minion is found. Do this a couple more times. Eventually, the other minions will come to associate the appearance of frogs with a horrible death. Sneak into the fortress and spread a few hundred frogs around. The place will go nuts! Panic like that may offer lots of opportunities for throat slitting.