depends on how you want to play. Do you want be a straver and use immo/explosion when it comes in handy. Or the other way around, using mostly fire skills and strave when it some in handy.
If you realy want to make a combo, using both often. I would do this*.
at least 6 in strave, so you'll have ten arrow shot. More on a single target is not needed if you won't use this as you bosskill skill.
Max immolation, to up the damage.
Max valk, I just love valk.
Enough pierce to get nearly 100% peirce. Start off with just one or two point. As gear some avable, you might have enough +skill to pull it of. Get raizortails.
1 in all Prerequisites, or more if you feel like it.
Some in ice arrow. This way you can freeze the boss so he stays in the immolation flame.
rest: where you like it, more fire damage, in synergies for immo. More out of strave, in strave self, or in critical strike or penetrate. Better freeze, more in ice arrow. Better valk, more in decoy or in the evading skills.
stats, as low strength as possible. Enough in live and mana, some skills cost a lot of mana and steeling is not always easy, and live is always good. Rest in dex.
If you want is on an other way, go with base mana, find a lot of mana leech. More points in critical and penetate. This way, you might be able to steel back the mana you spend on immo, or an other skill. And if needed, you can still strave to get it back. Max dex, for more damage, so more leech.
*: these are just ideas how I think about it, this is how I would do it, but I'm sure people will disagree with me.