The Scrake's behaviour and abilities are similar to the Gorefast's; it picks a target based on proximity, and may change target based on the actions of the players. After spawning, it walks at a slow pace towards their target. The main difference is its conditions for charging players, which is based primarily on its health, the difficulty of the game and a random variable; the harder the difficulty, the more likely they are to charge the players. Its modes of attack are very similar to those of the chainsaw that players can carry, and it favours the powerful swinging attack.
Strategy to kill Scrake:
On Normal, get a commando with a SCAR to single-shot mag dump into his head. Two mags, even with misses, will cut his head off before he gets angered, all the time.
Even the AK might work, but I've only tried one AK mag + one SCAR mag.
On Hard, the best tactic has been to have a zerker stun him with grenades, and then as many people as possible just mag dump in his skull. The explosive round of the pulverizer seems to do wonders at that time also. One of those + 2 mag dumps from 3 - 4 tier weapons should do.
you can dodge his lunge attacks to the side which gives you a couple seconds of him being in roughly the same pose while everyone unloads their guns on him as he resets.
You can also have a Support unload their shotguns into the Scrake. Use Double-Barrel and/or auto shotgun for best results.
For berzerker, get pulverizer and always click right mouse to use explosive attack, it's pretty effective.
If you have a sharpshooter let it use a rail gun on its head and it will clean it out quick or use a crossbow a couple of hits and it will be a memory and a body