ComboBreaker wrote:is painboy worth his cost,
If you use him well he is

350 req is a sizeable investment in T1, but as a subcommander he doesn't bleed at all, and he can help control your bleed a lot with his heals and support.
ComboBreaker wrote:when do you get him,
Like Torpid mentioned, he is great in the Ork and Nid matchups. In general, he excels at supporting melee units such as sluggas and stormboyz, since they have very high damage but relatively low health, he can allow them to stay in combat and provide damage output for much longer than normal, and win melee fights they would normally lose. He can also be used less aggressively to heal shoota boyz or lootas that are in cover and taking damage, preventing bleed and allowing you to win ranged firefights. he has some usage as counterinitiation against jump squads as well, you will be able to punish ASM or raptors fairly easily if you get lootas and protect them with sluggas/painboy.
in general I get him in several situations:
need more counterinitiation - I get lootas or heavy upgrades on shootas, and leave painboy and sluggas on the backlines as counterinitiation, using my ranged fire as the primary damage dealer. this can be good vs eldar or space marines, since they can heavily punish your melee pushes, but may not be as good at dislodging your ranged units. sluggas with painboy support can go toe-to-toe with ASM or banshees, which is a big deal in T1.
supporting melee compositions - with either double sluggas or sluggas+storms, getting a heal on those units while they are stuck in melee combat can be an engagement winner and allow you to get squad wipes. Torpid elaborated a lot more on this, but you can win most melee fights in T1 when you get a good heal off.
ComboBreaker wrote: when do you use heal(is it a good idea to use heal preemptively?)
His default heal gives back 30 health per model immediately, and then 15 hp/s for 8 seconds (heals 30 initially, and 120 over time, so 150 total). the up front heal is much less than the heal over time, which makes a big difference in how you use it compared to Apo/PC heals. Sluggas have 100 health per model (120 with burnas) and Stormboyz have 180 health per model. Optimally, you will use this heal on those squads when they are at 70% health(sluggas) or 84% health (stormboyz), but it is much more important to use it before you drop any models than if you get the maximum initial heal. so if you suspect that one model has been taking the bulk of the incoming damage, don't be afraid to use it early on.
I generally try to use it on sluggas while they are running in on a suppressed squad (from shoota boy/loota suppression) and have taken a bit of damage but have not lost a model. I tend to use it a bit later on stormboyz since they take less approaching damage when they jump in.
One big thing to keep in mind is damage distribution.
when units are in melee, damage tends to be distributed a lot more equally from the enemy squads since most models are engaged and dealing/taking damage. This means you can use the heal when your squads are at lower health, so that you get the most out of the heal over time. this can also bait your opponent into trying to commit to a squad wipe before you restore a bunch of HP and punish the overcommitment.
when units are being fired on from range (EG sluggas running up to get stuck in), damage tends to be more localized to the leading models. in these cases you will want to pop the heal earlier to prevent model losses. this is a lot more important than trying to optimize the heal over time.
However when healing shootas or lootas that are in green cover, ranged damage is distributed more equally since they tend to not have lead models (shootas more so than lootas). you can be a bit more ballsy when healing your squads in those situations.
ComboBreaker wrote:how do you position during engagements?
you dont want to blob too hard, but you want him near whichever squad you want him to be healing. i generally then split him off after popping the heal to tie up another ranged squad to lessen the enemy's damage output. he doesnt provide that much additional damage himself and its rare that you can heal twice in a single engagement, so he's better utilized in that role rather than as an extra slugga model.
One other thing to consider is that he gets exponentially more powerful in T2. He gets a much better heal with his first upgrade (50 initial health plus 30 hp/s for 8 seconds - 50 + 240 over time - 290 total), along with increased energy regen (can use heal more often) and durability (+100 health). However the bigger deal is definitely the Unstable Fuel injection ability. this is very similar to the Rokkitboyz ability that Stormboyz have (slightly more damage, less knockback radius), but it does not affect friendly units, making it absolutely bonkers to use in a melee fight (since you get so many free melee hits while the enemy models have been knocked down). Your sluggas will be able to win just about any fight you can think of when you use this as well as a heal, and it gets even more powerful when you combine with trukk support. It has a fairly low cooldown too so dont be shy about using it
