How to strengthen team?

[ShadowSlayer]

So I'm trying to get Michalis but my team is a bit too weak right now. Currently it's 

5* Tharja level 28

5* Clair level 25

2* Anna level 27

4* Lachesis level 21

The mission says level 30 and I failed pretty miserably when I tried it. But I'm not really sure how to make my team stronger. Should I just grind EXP in training tower? My Tharja hardly gains any EXP.

[MaskedAmpharos]

The Training Tower is the fastest way (aside from spending crystals, which I don't recommend because you gain SP less efficiently) to gain levels, yes. 

It's generally best to train one unit at a time (having a dancer and a healer makes this a lot easier) and pick a Training Tower stratum that's a few levels above the unit you're training. 

(Also for future reference, questions like these go in the Ask a Question thread pinned on the front page)

[Anacybele]

Firstly, upgrade Anna if you're going to use her. A 2 star unit won't cut it at all.

Second, you really want units that are lv. 40. Don't you have any?

Third, the only way to level up is to grind in the tower and do story chapters and stuff, yeah. The process gets slower at lv. 30 and above, but it's worth it in the end.

[Roflolxp54]

For GHB maps, you want to be overleveled or otherwise level capped before you attempt any of them. You should also see the star on the map name, which should tell you the overall star rating of the enemy team.

For example, the Hard Michalis GHB map is silver-star lv. 30. That tells you that all of the enemies are at 4-star rarity and at lv. 30. That 2-star Anna is not going be that helpful unless she's lv. 40 and got some useful inherited skills or you're trying to replicate Mkv's 3-star Lunatic Camus clear that used a lv. 40 2-star no-skills-equipped female Robin as a bait unit.

Training Tower is the best way to grind for levels. The reason that Tharja doesn't get a lot of experience is that she's at 5-star rarity. Units at 5-star rarity get the standard exp. gain while units at 4-star rarity or lower get increased exp. gain for defeating weaker opponents (likely to compensate for their lower stat caps when compared to 5-star units). IIRC, 5-star units get like 1 exp. if the target unit is 6 levels lower than the 5-star unit. And as MaskedAmpharos mentioned, it's best to train 1 unit at a time; a training team usually consists of the trainee unit, a healer (ie. Lissa or Wrys), and either 2 dancers (ie. 2 Olivias) or 1 dancer and a buff unit (ie. Olivia and Sharena).