Our team is really getting beaten by late game Medusa. She's now tankier than BristleBack and has the damage potential of Sniper. Are there any carry heroes other than Anti-Mage that can counter her? Effective strategies are also welcomed.
The #1 way to deal with Medusa is definitely either ending the game early (via a push-heavy lineup) or dedicating several heroes to ensuring Medusa gets no farm ever (and hoping her team has no second similarly scary hero).
Some ways to deal with Medusa include other forms of mana burn, such as diffusal blade, Invoker's EMP, Nyx, and Lion. Also noteworthy is Outworld Devourer, who steals intelligence with his Arcane Orb, which results in lower intelligence on Medusa, and thus a smaller mana pool for you to have to chew through.
The first strategy to defeating a Medusa is to counter pick her. How?...
By picking heroes that finish the game early(You can check for a list of good pushers on the dota 2 wiki).
Something to note is that when a medusa is picked, her team needs to compensate for it by getting heroes that are strong early to mid game(this is similar to when an Anti-mage is picked) as they will be playing 4v5 for 1/2 of the match's time. If they do pick so, then you should instead focus on hindering Medusa and supporting your carry until late-game(at which point the carries come online and other cores start to fall off in power) to allow your carry to come online earlier than their carry.
You can use a wide variety of heroes to gain an advantage, I'll list a few prominent heroes but you should consult the wiki on further details.
Popular pushers are: Lycan, Meepo, Lone druid, etc.
Popular Harassers are: Bounty Hunter, Riki, Pudge, etc.
Popular(albeit not as powerful) counter carries are: Anti-mage, Phantom Lancer, Naga Siren, etc.
Some tactics to use on certain heroes are:
Medusa dispells all illusions that become petrified, this can be countered to some extent with PL and Naga.
PL can cast doppelganger as the illusions are about to be petrified(the petrify counter does not stop at all for hidden and invulnerable heroes), the petrify debuff will fail to affect the illusions since they are hidden and the counter will reset.
With Naga, it is slightly difficult; you have to cast mirror image just as you're about to get petrified.
Phantom Lancer can use diffusal blade to an extent but the amount of illusions he has to spawn to make it less effective, because Medusa can use her ultimate to petrify them(which insta-kills illusions) or Split Shot and Mystic Snake.
These interactions have not been tested by me, I am theorizing this from information in the wiki.
Finally, I would like to suggest you look at this page as well.