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"Enemy mobs and bosses will spawn based on a 'signature' task force's maximum level (and the leader's notoriety setting) regardless of the team's composition. Therefore task forces are usually done by characters at (or above) the top of the level range. "
Posi is a signature task force, therefore enemies will always spawn at the TF's max level, which is 15 + or - the difficulty level.
Difficulty level 1 would be what it is run at.
On Heroic/Villainous, you'd get a normal amount of L15 and L16 mobs, regardless of team member levels. Also note that if you start with eight, you'll get mobs spawned for eight even if seven of them log off (members must actually quit or they'll spawn enemies as if they're still there). On most of the successful Posi TFs that I have run, the players have been over, at, or very close to the level cap...huge mobs of +5 and +6 enemies make for slow going at level 10. The good news is, if you can hack it, you won't be level 10 for long. Good luck!
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In that case, you will see all 15s and 16s, regardless of the levels of the team. I do think that the larger the team, the more 16s you'll see vs 15s, but it still does not depend on level of the team.
Unless you are looking for a LONG challenge, I would not recommend trying Posi with a team of 10s nor with a mixed team where max is 13. Wait until at least half the team is 14+. Personally, I always exemp Posi on small teams for my toons that need Task Force Commander.
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If the team is not too large (5 players or less), and they are prepared for the first couple of missions to take longer, then the players can pretty easily run this at level 12. The key is to avoid a group size that generates bosses in every group, this is what will slow you down and kill you more often.
That kind of team makeup should gain a level per mission or so and from there the rest of the TF will be pretty simple.
As for what is on your team, go with ATs and powers you probably wouldn't expect.
- Avoid too many melee types (they are missing key powers and slots that early)
- look for Defenders and Controllers (but don't overemphasize healing)
Radiation or Sonic are good choices, as is Dark. Plant Control would be awesome as that is one of the most crazily front loaded sets in the game. A Plant troller (or 2) with Seeds of Confusion and the AoE immobilize can effectively tank for the team.
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Part of the challenge is the bosses. It almost sounds like this should not be tried until lvl 13-15, or lvl 10-12, but a smaller group.
If I have 8 lvl 10's, are the baddies lvl 10 or level 15?
If I have a mix, and the high is lvl 13, will the baddies start at that level or be 15.