Saw on the forums (yes, I went to the official forums. I was bored, ok?) a good analysis of the way missions' chances are calculated.
Basically, a mission has 3 penalties per attack, plus 1 per enemy. You get 3 advantages per countered attack, 1 base advantage per follower, 1 advantage per bonus ability (terrain/targets, Blook's Combat Experience, High Stamina) and 1.5 advantage per racial bonus or Dance if there's a Danger Zone. A lower ilvl reduces all the advantages for that follower, a higher ilvl increases the follower's base advantage. Get enough advantages to counter all the penalties and you get 100%.
So, if you have a Highmaul Raid mission, there's 6 attacks (18 penalties) and 3 enemies (3 penalties) for a total of 21 penalties. You need to provide 21 advantages to guarantee the raid's success, with any combination of advantages (countered attacks count once, I think terrain/target boni only once because you have the "green tick" showing, but I've verified that all other stack).
So basically, when building a team, you need to realize that each attack that lacks a counter can be compensated by:
- 3 boni (terrain, Experience, Stamina)
- 2 racial advantages or Dance
- 45 ilvl of gear on the followers (like it's going to happen for 645 missions. Best is 30ilvl)
- OR a combination of the above, like 1 boni+1 racial (1.5) +8 ilvl (0.5)
So, if you're planning to get your followers ready for raiding, the main point is to raise ASAP to 645 any epic follower with:
- Timed Battle
- Deadly Minions
- Wild Aggression (those 3 attacks occur in 3 of the 4 raids, and twice on one, while other attacks occur only in 2 or a single raid)
who also has Gronnslayer, Ogreslayer, Mashwalker/Evergreen, Plainsrunner, High Stamina and possibly a racial preference with any "designated" team member. Don't bother getting above 645, you gain 1/15th of a counter by raising a follower 1ilvl above 645, but you gain from 4/15 to 12/15th of a counter by raising a sub-645 follower to 645 by 1 ilvl. And yes, Blook is almost certainly, if not the first, one of the first priority because he starts with Timed Battle AND Combat Experience before getting Rare/Epic upgrades, which means he's probably going to end up in your 645 team.
Tactics when a raid pops and you've got basically to figure out how to tackle it because you don't have 100% yet (you really need to install Master Plan to manage your raids):
Interacting with the Master Plan suggestions, pick the highest chance team and see if any of the members is below 645. If there is, funnel all your tokens to him/her before sending your team.
If all your followers for best chance are already 645, then pick the second best team, and see if you have more counters active and/or more buffs active than your main suggestion. If you have, then see which followers are below 645 and have the most relevant counters, and funnel your tokens to that one. If not, see the third offering by Master Plan.