First - I know some of my players frequent this forum, and I would ask them to please stay out of this thread, to spare yourselves spoilers.
Now that's out of the way, onto the post. I'll be requesting advice for this mission, but first, some background.
My group has just started their last mission as a Deathwatch Kill-Team. We'll be moving on to Black Crusade after this mission has completed. My players are all Rank 7~ with 80+ Renown and all kinds of gear (I awarded 200 points of Requisition for this final mission). Their previous mission saw them board a Chaos Space Hulk with the intent of retrieving a Dark Angels relic, with the result that they became trapped in the Warp for over 70 years (this becomes important later).
Alphus III was, until recently, an Imperial-held world. Representatives of the Tau managed to brainwash the people into accepting their rule with their message of 'the greater good'. The Tau have so far managed to beat back what Imperial forces remain, as well as blockade the planet against further incursions. The Kill-Team's mission is twofold - assassinate the Ethereal leading the Tau troops (bonus points for doing it in a particularly demoralizing manner), and 'purify' the world's inhabitants - by whatever means they deem necessary.
To accomplish their mission, they were instructed to work with a puritan Inquisitor in the area, who was deeply suspicious of the Kill-Team (based on their prior 'associations with Chaos'). The Inquisitor's secretive nature, coupled with his inherent mistrust of the Kill-Team, caused the team to become angry with him. They have taken over command from him, much to his displeasure.
Under the Kill-Team's command, the Imperial Guard managed to break the Tau blockade and land many of their reinforcements. They used the Imperial Guard forces as a distraction while the Kill-Team infiltrated the Tau-held city and began to take out their AA (Anti-Aircraft) batteries. However, after the Kill-Team managed to destroy a pair of the batteries, they noticed movement among the night sky - Ork dropships. The Kill-Team made the decision to cease-fire against the Tau until the Ork menace was dealt with, negotiating the 'truce' with Commander Shas'El'Vrai.
Now that the plot exposition is out of the way, we get to the meat of this post. We'll be shortly seeing Tyranids, Eldar, and Necrons dropping into this scene. Additionally, I want to grant the Kill-Team reinforcements in the form of a Space Marine company - possibly even a Titan (just for the sheer badassery of it).
So - questions:
1) I'm pretty new to running 'freeform' missions. Any tips?
2) If I give them a Titan, what can pose a credible threat to it, given the xenos I've outlined here? Would prefer Ork, 'Nid, or 'Cron threats, as I'm planning for a pretty light force of Eldar.
3) Where can I find some Eldar or Necron stats? If possible, would prefer stats that are compatible with the 'new' Necrons, as I love all the new options.
4) How should I handle the Tyranid 'Shadow in the Warp'?
Any and all advice is appreciated!