Up to date f2p guide

BTW, I stopped leveling mods with fixed or unnecessary primaries on lvl 12, because I also have credit shortages.

That is a huge mistake my friend. Fixed numbers are the best stats you can get on secondaries. % stats on mods become a bit stronger when you level up, but overall they wont outscore those giving fixed stats. Use swgoh site to navigate. For example: Maxed out Lando defence is near 200 - 2% secondary gives +4 to it, while you can easily get a +10/12 stat on raw number. Same goes for ofence, Lando has 2,1k dmg so 2% offence will boost it by 41. And currently I got a +60 offence secondary on a 3* mod. Take into consideration that I talk about maxed out units, so it's even more noticable on lower levels. If you don't believe me check it yourself while equiping/unquiping mods. Same thing can be said about protection. Other stats are different, because base is higher or they add flat %.

I find this post very useful. I'm ranking around top 10 in arena and about halfway through farmings Lando to 7*. For my team, I change the lead between Lando and Phasma; so my question is given I lead sometimes with Lando, so I start to farm Biggs instead of GS given Biggs is a rebel and can benefit from Lando's Leader speed and crit dam? Both Biggs and GS are currently 2*

Well, Biggs and GS are in separate squads so I don't see the problem with farming both.

Actually, I think I will stay with GS as he can be upgraded so all allies have additional +13% critical chance - perfect for Lando with groups :)

I have no idea what you mean by that. GS is a standalone blade. He rarely benefits from anyone and helps only geonosians.

Right now I'm level 62 and usually finish 2nd or 3rd (I can't beat number one who is lando, ackbar, boba, then sth and scarif rebel Pathfinder. Which is weird but I can't get through his tanks to take out his threats) , so any tips there would be great.

I would only say to rejoice, you got an easy arena bracket. You'll benefit immensely throughout whole game. I'm 62 also. I would decimate your top1 w/o any problems. On the other hand my top1 is over 70lvl with wiggs/chaze/lando and top 10 is crowded with Lando/palpatine/chaze/Boba squads. You literally have no idea how lucky you are. In the long run it's few thousand free crystals you'll get compared to others who have to battle whales in their top10 arena brackets.
Not much can be said to help you out since we have no idea about your/his gear/mods. Not to mention your team composition. Sry, but I'm not a psychic.