Scoundrel suffers the following detrimental impacts to it:
1) Some abilities rely on Str, others on Finesse.
2) Most skills require using a dagger, despite that being the Worst weapon to auto-crit with (due to low base damage, ideally you'd be using a two handed weapon).
3) Most of their CC is resisted by Magic Armor, despite most of their attacks dealing physical damage.
Adding onto that skills that rely on Int, for example, is just going to lead to an ineffectual character torn in too many directions to be useful. So everything Int is out. For the same reason imho everything Ranged is out, cutting out virtually all of Marksman.
What's left, then?
- Movement abilities from Aerotheurge
- Summons
- Melee combat abilities from Warfare or Polymorph
- Self Buffs from Pyrokinetic (such as haste) and Polymorph
Polymorph is almost certainly the best, as in addition to giving self buffs and melee abilities it gives attribute points as you invest in it. Warfare or Aerotheurge are certainly viable and interesting options for alternate combat styles and combos, with Warfare focusing more heavily on the CC angle (both because you get their physical armor down faster to CC them and because it has physical armor based CC all over), while Aerotheurge is a strike and retreat style. Pyrokinetic is more of a dip, needing only a point to get anything you would really want from there.
Summons are fantastic, and certainly there's nothing wrong with someone sneaking around summoning stuff, staying invisible, and providing leadership boosts to the allies around them, but honestly Scoundrel is a sub-optimal second tree for a summoner and thus while it's one of the Better pairings for a Scoundrel, it's one of the Worse pairings for the Summoner and I'd not recommend it.
