Below are some ugly things you might face in games as well as how to counter them.
1. High Resistance Heroes (Dragon Knight)
Recommended Counters: Obsidian Destroyer (Pure damage), Slithereen Guard/Dazzle/Tauren Chieftain (Armour reduction).
Other Counters: Silencer(Pure damage), Witch Doctor(Maledict), Necrolyte(ulti), Chen/Pugna/Undying(Damage Amplifiers)
Item Counters: Desolator, Orchid Malevolance
2. Invisibility (Rikimaru)
Recommended: Bounty Hunter/Slardar(Track), Zeus(Bolt gives truesight)
Other: Spiritbreaker(charge gives sight), Alchemist(stun gives sight), Bristleback(Reduced back damage)
Items: Gems, Sentry wards, Dust of Appearance.
3. Mass Disable (Lion, Rhasta)
Items: Black King Bar (Magic Immunity).
4. Jungling Carry (TerrorBlade)
Recommended: Beastmaster/NerubianAssassin(solo ganking)
Other: PotM/Spectre(Semi-scouting/Semi-solo ganking), Lion/Lina/Leshrac (semi-solo ganking), Luna/Juggernaut(Ganking carry)
Items: Warding jungle area.
5. Buffed Carries (Omniknight)
Recommended: Medusa/Razor/ImageHeroes going for diffusal(Purge)
Optional: Chen/Enchantress(Purge by controlled creep), Mangix(Split’s semi purge)
Items: Diffusal Blade(Purge), HotD(Control purge creep).
6. Mass Ganking (Beastmaster, Vengeful, Zeus)
Recommended: Weaver(Watchers), Beastmaster(Scout), Treant(Eyes), PotM/Chen(Helping with ulti)
Optional: Furion/PitLord(Teleport when needed)
Items: TeleportScrolls, Observer wards.
7. Strong Summons (Warlock)
Recommended: Obsidian Destroyer(Orbs bonus damage)
Optional: Chen/Enchantress(Convert strong summons like beastmaster’s pig and neutrals), Earthshaker(more summons, more damage)
Items: HotD, Diffusal Blade (High damage to summons).
8. Mass nukes (Puck)
Recommended: Nortrom/Krobelus/Traxex(Silence), Omniknight(Repel), Pugna(Nether Ward)
Items: HoodOfDefiance(Damage Reduction), BlackKingBar(Avatar), Bracers/Str.Treads (more hp).