Against Human
Archmage first, build as many Spellbreakers as possible and add a few Priests. Second hero MK. Since the enemy probably builds the same units your best chances to win are either better micro, higher hero levels, a lucky creepjack or a perfectly timed attack on the base, when you know that the enemy is not there. If an expansion is possible to build and hold long enough that you actually make profit from it, then build it.
For lvl 20 follow this instructions!
For higher lvl, play Am with some footies, t2 add mk and one arcaine sanctuary. On your way to t3 you add a workshop ( blacksmith needed ) and built some mortars against the enemy casters. Your army should consist of about 3 Priests on inner fire and about 5 or 6 for healing(use the inners to dispell if needed), some mortars to kill the enemy casters and knights! Of course you can add a Paladin when you are at t3. Yo can use the mortar teams to scout your enemy and check out if he is trying to get mass air, if he tries so, you can make an expansion or attack him because of your advantage. If this doesn't work make Dragonhawks as air conter.
Very important in hu vs hu is very good micro in the early game. When you fight with Am, we`s and footies vs your enemy with the same army,it all belongs in your micro. The winner of the first fights is probably able to tech faster and that's what's really important in this matchup!
Against Orc
Archmage first with footies. Build an early blacksmith(while teching on t2). Build rifleman and a few Sorceresses( 3-4) and add a few Priests. Buy a Beastmaster from the tavern as second hero and try to win through the summons, start an attack to the burrows. Try to level your heroes as much as possible and attack before the enemy TC reaches level 3 or the Farseer reaches level l4. Because if that happens his Shockwave/Chainlightning will kill your Sorceresses and Priests in no time(Healscrolls solve) . Only build an expansion if you are 100% certain that you can hold it (Lost Temple), else it delays you too much which gives the orc time to level his heroes.
Alternatively you can pick the Mountain King as second hero and skill bolt. if you have enough rifleman and maybe sorcs, you should be able to focuss single units.
Against Night Elf
You use Archmage as standard hero. At first you build footies, cause most night elves will produce archer. The footies make extra damage to the archer, so they are best but you have to micro them, the attack damage of the archer is not bad at all. If the NE goes huntz in the early game, you have to switch to rifles soon. A blacksmith, necessary for rifles, should be build anyways. The second hero can be the mk, with bolt and more important clap, he can slow enemy units, and then the archmage can use blizzard, which can be retrained. But clap is good anyway, because it slows the enemy attack. The T2 units at the human should be mass caster and then breaker or rifles...
Another tactic can be knights and mortar... You build same heroes and this special units, instead of rifles and only a few caster or even no caster... Early game is the same...
Against Undead
Archmage first, build a few footies for easier creeping and tech to tier 2 ASAP. Build mass riflemen, a few priests and maybe 1 or 2 sorcs with slow autocast set to off. The undead will most likely play mass destroyer, so sorcs will be useless any way, but there may be some situation where a manually casted slow will allow you a hero kill, or where invisible will save your hero. Try not to force the win through an attack on the base - undead bases are strong. Also don't fight if the destroyers have mana. Try to win through a fight in which destroyers have no mana, so your riflemen should rip them apart. If the enemy is attacking your base, let him waste all his mana on buildings which you should mass repair. In some situations an expansion is possible even before the enemy has destroyer. If you manage to do that, and if you manage to hold your expansion against the destroyer attack, you will soon have a huge army of riflemen, which will make things a sure win.
2on2 with Human
This strategy applies for 2v2 maps; both teammates must be humans: Start off by sending 2 of your peasants to mine gold while the others build an alter, one barracks and one farm. This enables you to train a peasant right away to send into the gold mine. Once the other peasant has finished the farm, have him build another (this saves money). When you finish building the alter get the paladin (both teammates) and then start training footmen as soon as you can. Have the peasant that was building the barracks search out your enemy (just one teammate; it's advised that you avoid orc at all costs (but if the blademaster attacks you too much, run to the orcs to distract them). If both enemies are orc, when attacking go for the burrows and the peons. The first attack should be focused on getting your hero to lvl 2-3.
You should have footmen on the way; never queue up more then one at a time or you'll end up putting your extra gold to waste (you want a pool available at all times). You need to have 5 peasants on the gold mine, 5-6 peasants on trees and maybe 1 or 2 for building (or hacking at wood while idling). When the paladin arrives both teammates should get Holy Light (to heal one another) as soon as you have three footmen, research the defend ability. Half way through its research time, go and attack the enemy (avoiding orc if possible). Attack one base together, as this enables you to heal each other. Enable the defend ability to prevent any range from harming your units too much (don't forget, this means all range including towers and ziggurats). As soon as your hero levels up, get the invulnerability spell. Always have footmen training and running to your hero while attacking the enemy. To do this effectively, set the rally point on your hero. It's also advised that you put a hotkey number on the barracks (press Ctrl + 1 , or a different number of your choice).
If the attack fails, run back to your base and upgrade your town hall to a castle (if it is possible while you're away, start upgrading to a keep) as fast as you can. Make sure you build a blacksmith for researching upgrades. One teammate needs to get a small amount of rifle men and knights, 5-7 mortar teams, some priests, and spell breakers if necessary. The other teammate should get about 6 rifle men, and 7-9 or so knights. It's recommended that your second hero should either be an archmage or a bloodmage (or the darkranger, but don't forget she is undead so you cant heal her with the paladin's holy light). Sorcerers, healers, and spell breakers must come in abundance (meaning the other player who is not getting mortars needs to get large amounts of them).
Your Paladin should be level 3-5 and your second hero should be level 2 or 3. Feel free to get a third hero of your choice but you needn't bother because 2 heroes is normally enough. You should be ready to either defend or launch an assault but make sure you have all the upgrades for your units and your blacksmith should have only the last upgrades to go. Don't worry about researching them. Focus the mortars on either units with range or magic powers, or you can focus them on buildings of use to the enemy (like towers) . You should have the enemy at their knees in no time. If they mass air, don't worry because they will fail (magic and ranged units will take them down easily). Try and prevent your magic units from dying to soon.
2on2 with Orc
2on2 with Nightelf
2on2 with Undead
FFA
Don't worry about smaller units; go big. Few Footies to creep, get the Paladin for your hero (great with footies to tackle hard creeps) and tech to something like gryphs and a few hawks. Build some towers (no more than 4). Use the hawks to disarm all enemy towers, and you can raid a base in no time.