Retribution/Bloodletters/Strategy
This is a simple bare bones post because nothing is here yet.
Basic Tactics
Bloodletters are very strong offensive units but unable to take much of a beating if they are the target of concentrated fire. with this in mind, using them can be similar to using heretics offensively, allow the enemy to acquire a stronger target, like in cover marines, then either send them in. However, unlike Heretics, the bloodletters' abilities make it much better at crossing the distance. Teleport lets them go straight into the enemies units, without running the risk of enemy fire taking them down.
A quick scenario: you have two marine squads, two heretics, one bloodletter and a chaos lord. your fighting an ork hoard, two or three sluggas, and two shoota boys, and a war boss. Move your marines into cover, then once the enemy has targeted him, first move your tics and lord in to engage their melee, then warp you bloodletters into the enemy shootas. This is where bloodletters really shine, they are able to massacre most ranged units due to their high damage, and their low health is not much of an issue. whether you ultimately come out on top in that fight depends on variables, but the point i'm trying to make is that Bloodletters are the end all unit for taking on powerful ranged units. I have even used them with some good effect to take out powerful ranged vehicles like tanks, when they don't have support. the strategy is essentially the same as for ranged infantry, but make sure to warp behind them so you get the rear armor hit bonus.
That is not to say, however, that they cannot hold their own in melee against other melee units. Especially when supported by heretics worshipping, which makes their health regen much quicker. The best heroes for this are, in my opinion, the Chaos Lord and the Plague Champion; with the Khorne worship the BLs move faster [speed 7 instead of 5] and this is useful for closing the gap or to avoid everything, even a Bloodraging Khorne Dread. Last, but not least, try to use 2 BLs vs a Baneblade, teleporting them right behind the IG Tank, and activate the Global named Bloodlust. The Bane will be destroyed by the 2 BL packs in a matter of seconds. With the Nurgle worship the BLs can heal even faster. The Tzeentch worship will cloak the BLs so you can use them for ambushes, but in pitched combat it is less effective.
Countering Bloodletters
Tactics wise, be careful to spread out your ranged units, and if they teleport in and attack your ranged units, concentrate some fire from your other ranged units on them. do not try to run unless you can immediately put some distance between your unit and the Bloodletters. They are fast enough and do enough damage they will probably kill a couple of your units before they can out run them, so if you have only two or three people left and no support, they're gone. Better to try to hold them long enough to get some other units on them. If you cannot get enough fire in them quickly enough, you can probably count you're unit as dead, unless you make them retreat instantly. In pitched battle, when most of your units are tied up, they can, as I said above, wreck whatever they set their sights on, so be vigilant, especially when you have a large mid-game battle. The Bloodletters are ultimately best against the undisciplined army and absentminded player.
The best unit to units to counter with are other high level melee units, and especially ranged units supporting them. If you tie them up with Warriors, nobs, or even sluggas or banshees, then pour on the fire you will probably take out the whole squad, a large loss for your opponent.