start by making your base and surrounding the entrances with turrets.
build some power supply and capture all reqisistion points nearby.
everything should be protected! build your army until you can make terminators .
destroy all of your units (squad cap) and make as many terminators as possible.
upgrade them with flamers add comanders and have an orbital relay ready just in case (ive not had to use then yet) fill it with dreadnoughts .
as soon as you go into battle take the unit as close to the enemy as possible. the flamers will tear the units moral to shreads. this setup will destroy any enemy unit in seconds

It really doesn't matter if you have flamers at all, Termies will kill anything in a few seconds so long as ther're supported. this is a crap ass tactic...I would highly advise anyone to not try it.

HAHAHA, I would just like to see you try this against something like a Baneblade backed up with 2 Leeman Russ tanks and the 3/4 Hellhounds

So what's a good kick azz strategy then?

If you mean ONE unit, I think of a "Master Unit" like a Baneblade, Squiggoth, or Awakend Monolith...

But, if you mean a most powerful army, my advice is:
A "master unit" + some powerful vehicles (like Fire Prisms, some walking machines - not too many and heavy artillery.) + 2 to 3 squads of melee heavy infantry (for intercepting enemy melee infantry and as a "fire shield" for other units) + 2 anti-vehicle squads (for quickly destroying enemy vehicles before they come to other units) + your strongest ranged units (the rest of squad population cap).

The army should be fully upgraded, squads should have attached support units and heavy weapons.

example for Eldar: Avatar + 2 Fire Prisms + 3 Wraithlords + Warlock squad (with Farseer attached to them), 2 Banshees, 2 Dark Reapers, 2 Fire Dragon squads + 3 Warp Spider squads (for a quick surprise attack by teleporting behind enemy's main army to kill their support units)
It's good to have a unit with abilities witch devastate a wide area (Farseer-Eldrich Etorm, Force Commander-Orbital Bombarding, Imp. Guard Commander - Strafing Run, ...)

all you need is a few bonesingers and a dark reaper squad or two. build four turrets upgrade them to d-cannons then teleport your bonesinger into the enemy base while he isn't looking then build a webway gate and relocate your d-cannons and whatch the destruction. just make sure you remember to cloke your webway gate and turrets so the enemy can only retaliate with detector units. this is where the dark reapers come in. if the enemy brings in detector units bring the reapers in through the webway kill them and then back out.