Human vs Orc
Orcs have always been a bit weak against casters, as they have no real anti-caster unit, and no magic immune unit either. Humans can use this to their advantage with sorceresses. Also they tend to be weak against air when not given time to mass batriders. Orcs generally play 3 different strategies against human:
Farseer, TC, shaman, grunt
Farseer, SH/Naga, wyverns
Farseer, TC, grunt, raider, catapult
Note that orcs can always mix it up a bit, for example raiders and shamans, catapults and raiders, wyverns and raiders. Same goes for the heroes.
Standard rifle/caster can work vs these strategies, although you need to improvise a bit to counter them effectively. If they go wyverns, you will obviously need more riflemen than casters, as casters die very fast and deal almost no damage against wyverns. Priests aren't as effective as the wyverns will mostly focus fire each unit. You might want to skip casters completely and go for dragonhawks instead. This forces the orc to get batriders which will reduce his number of wyverns, giving your riflemen a much easier time.
Another tactic that quite some humans use here is mass casters with 2 summon heroes. The big amount of magic damage coming from your casters can easily overcome grunts, and wyverns die fast to the summons. Watch out for catapults!
Most humans get an Arcane tower at the start to counter the harass, otherwise the orc can keep sending wolves to rip up some peasants.
Depending on the map, you can opt for a fast expansion. Orcs can generally harass very well, so this is a risky option.
Make sure you don't allow the Tauren Chieftain to reach higher levels than your Mountain King, or you could be in trouble.
If the Orc gets shamans, you might want to consider retraining your Archmage to give him blizzard, as your water elementals only give free experience. Don't buy the tome too early however, make sure you have scouted him. Blizzard is not very effective against wyverns, and ensnare from raiders cancels the spell.
Lately Orcs have been using the Blademaster more, so i'll add a little bit about this. You definately need an Arcane tower in your main base to keep a BM from harassing you, but as you also need it against a Farseer, you will probably always get it against Orcs anyway. Always get a Mountain King second against the BM, as he is best at killing him. Try not to allow him to kill too many units, because a high level BM is very dangerous. Slow and storm bolt, with a dusk of appearance are the main counters against him. A Beastmaster's hawk can be usefull to detect him but i don't think it helps alot in battle, as you really need to stun him or slow him down in order to kill it.
Gryphons are effective against Orcs, but don't mass too much air. This will make his batriders more effective and leave you vulnerable on the ground. Instead, get only a few gryphons and add dragonhawks if he tries to counter with wyverns. As ground add some knights to hold the line while the gryphons deal the damage.
Human vs Night Elf
Night elf has the strongest harassing abilities, and Human is the weakest race against hit and run. So that is usually what this matchup is about. Night elves will generally try to kill as many units , and steal as much experience as they can while building up an army that counters yours (bears).
I could name all possible heroes/strategies that Night elves can take here, but the list would be very long so i won't. Night elves are the most flexible race of all, so you will have to improvise alot. Instead i will talk about possible human strategies.
A fast expand is possible, but he will undoubtly harass with hunts. Huntress beat up footmen, militia, are very fast and they can hide. This makes them perfect harassers against early game human. Coupled with a Demon Hunter with immolation you will have a hard time getting your town hall up. If you do get it up, make sure you put up at least 2 towers to keep him from harassing.
Tower rushes can work too, but are generally beaten off by archers. By sending an early scout you can find out if he is close to you, something you will want if you are going to tower him. Try not to fight inside his base, yet still try to keep the archers of your towers. When the first tower comes up, and your footmen and hero are still alive, you have probably won the game.
Many Night elves go bears against Human, so if you're not planning on either of the previous strategies, you should prepare to counter them. My favourite tactic is to hit and run him myself when i just reached tier2. He will probably try to get up 2 Ancients of Lore, which are your main targets. Make sure you buy a tp everytime you go for a run into his base. While delaying him, you should be getting some casters, and at least 1-2 spellbreakers. As a 2nd hero, you can add either the Bloodmage or the Mountain King. When he finally does get his Lores up, attack him in full force, preferrably with towers too. Use the spellbreakers to steal any rejuvenation he might cast on his heroes.
Knights and gryphons can also be used to fight off the bears, however he can counter this by adding more dryads. In turn you can get mortars, although this requires quite some buildings, so you will need to make sure you have a better income than his. Overall i think this is the most interesting matchup.
Human vs Undead
In my opinion the most balanced matchup beside the mirror match (I try to refrain myself from calling imbalances). Every undead will undoubtedly get tier3 as it is pretty much their only way to beat humans. The most common tactics for undead are:
Mass ghouls with a few destroyers
Mass abominations with a few destroyers
Mass fiends to destroyers/abom
As you can see the undead pretty much always adds destroyers, which are without a doubt the most dangerous units for Humans. As for Undead heroes, you will mostly see Death Knight/Lich with possible a Crypt Lord as a third. Also possible is the Dreadlord when he wants to fast expand (most common on Lost Temple as a counter to the Human fast expand).
Allowing the Undead to reach a solid tier3 army without punishing him for it means you will probably lose the game. You can punish him by harassing/pushing his base to pick off as many units as you can, or his acolytes while he is teching. Another, and this is the most common, is getting an expansion as soon as your Mountain King comes out. As the undead is still at tier 2, your water elementals and MK will be capable of taking out the ghouls. Make sure you stay near your expand so that he can't kill your workers. When it is up, add at least 2 towers with a masonry upgrade to be able to withstand the tier3 attack that undoubtedly comes. Use the time in between to level up your MK to make him more dangerous and tougher, and to make sure he doesn't expand himself.
As far as army choices are concerned, rifle/caster is strong until he gets too many abom/destroyers. In this case you will have to outmass him, and try to kill his heroes. Make sure you have enough healscrolls if he has impale/nova, and teleport out as soon as you run out of healscrolls. Do not underestimate the power of his heroes, they can decimate whole armies at higher levels.
You can also mass air, but only do so if you have at least 1 expansion up, otherwise you will not be able to fund it. Getting a Paladin as a third hero is good as well.
One thing to note is that you must always have a Scroll of town portal with you in this matchup. Not only will you lose alot of units if you're forced to run, but alot of Undead also tend to make raids on your main base or expansion. If you see him coming into your base and you know you can take him with your army teleport in as soon as possible to reduce the amount of peasants/buildings lost. If an Undead does this alot he is probably trying to get your attention away from an expansion he is trying to get up.
If the Undead goes fiends, do not get riflemen early on. Instead get 2 Arcane Sanctums and mass breakers with a few sorceresses behind it, along with defend footmen. As soon as you are sure your army is stronger than his, you have time to put up an expansion. He will probably counter you with tier 3 now, so add riflemen and possibly some knights.