Tactics is an option in Doom & Destiny where you can switch between a set of passive boosts/debuffs based on which of the four Heroes you select as a Leader and whether you put each Hero in a forward, middle or back position. The Forward and Back positions give a strong boost to one of two stats based on the Leader, but a strong penalty to the other so you have to consider the tradeoff, whereas the Middle position gives a very small boost to both. The strength of the boosts and penalties increases based on the character level of the Leader. Each Hero's boosts/penalties as the leader will be evaluated below, but will not include the weaker-effect Middle position. it should be noted, that when you get to a higher level around 30 or so the debuffs for whichever positions you choose are completely removed and instead start giving you benifits for both stats in all three positions.
there is roughly 5% Exp. bonus for the leader (rounded down).
He can be a risky choice for the leader for either position. The members will be either a glass cannon or a walking tank armed with a toothpick. Only the very magical Nigel can get a fully risk-free boost with the HP one since his main attack power doesn't rely on Strength at all.
Choosing the position for high Crit Chance is also risky like the high Strength option for Johnny; true you will land highly damaging critical hits more often, but the status effects inflicted by enemy units can really mess with you. However, going the other direction and getting high Resistance at the cost of critical hits, although an annoying cost, isn't a crippling one and being able to heavily resist status effects can be very handy.
Honestly only Nigel himself and perhaps Mike can really benefit from this one and the rest from the middle position for a minor boost to both. The MP pool of the other characters is likely to be far too low already to risk a hit to it and their main attack strength doesn't come from Spirit.
Choosing the Evasion boost/Speed penalty position is a catastrophic mistake as well as self-defeating. Sure your heroes may be able to evade attacks a bit better, but being notably slower the enemy gets more chances to try and your Evasion will never be perfect. Having a very low speed is absolutely crippling by allowing every enemy on the field to not only get the first strike on you - including all potential debuff effects - they may even be able to each get multiple hits on you before any of your own characters can move, and for every subsequent move afterwards.
The reverse, however, is a very different story. Although completely removing all Evasion from a Hero is annoying it is by no means crippling; quite a few combat games don't even have a Dodge mechanic and it shouldn't be too difficult to accept that you'll just take the normal-strength hits from the enemy. And in a roundabout way having a high Speed can actually eliminate the penalty by letting you kill the enemies before they even get to move in the first place. If you put Francis as the leader and have all four Heroes in the Forward position for high Speed you can completely control the flow of every fight. Carefully check the timer bar at the top and plan out your attacks, and you can heavily cripple or kill many of the enemies on the field before they can get a single attack out. This becomes even more powerful if you equip as many Heroes as possible with abilities that Stun (the little swirling yellow stars) which knocks them backwards in the time bar; with high Speed and repeatedly spamming the stunning skills you can potentially permastun an enemy for as long as the MP lasts on your Heroes, doing utterly catastrophic damage and preventing them from fighting back.
By letting you hit first, and hit more frequently, the high Speed boost of Francis effectively gives you the Spirit boost of Nigel, the Strength boost of Johnny and both the Crit and Resistance boosts of Mike. They'll cause damage more frequently and heal more frequently. You also get the benefit of being able to ignore all passive powers and equipment that boost Evasion, and focus on building up your other stats to a much stronger level. From a tactical viewpoint having Francis as the leader and all four Heroes in the high Speed position is the most powerful and effective formation.