Leveling for ME:A multiplayer works similar to Dragon Age Inquisition's multiplayer co-op. But for those not familiar I will attempt to explain below.
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Each MP character has 5 skill trees and max level of 20.
After level 20, your MP character can continue to play as well as respec their skills and at the same time earning points towards the account wide bonus stats.
Players cannot unlock all skill trees. Only a maximum of 85 skill points.
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Back to the 5 skill trees, there are 6 tiers with each tier unlock costing (progressively increasing) skill points. To completely unlock an entire skill tree, players must invest 1+2+3+4+5+6 = 21 skill points.
And each player has only max level of 20, with each level rewarding 3 skill points for a total of 60 skill points earned via leveling.
That means there are 25 skill points 'remaining'. Where do players get those 'remaining' skill points? Via store Supply Packs (using mission points and alot of RNG spinning) to obtain random Character Cards.
Each (even-numbered) Character Card unlocked, will also reward 5 skill points. For a total of 25 skill points.
So please wisely plan your skills trees. Combos include: 6/6/6/6/1 or 6/6/6/5/3 (with 1 skill point not used)
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My Supply Pack strategy (suggested only):
Initially focus on Basic Supplies. This will help in unlocking Common characters cards (increasing chance of characters to get more skill points or hitting their max skill points), unlocking Common weapon / mod cards (which helps to improve weapon and mods) and adding a small amount of consumables (Revives / Health / Ammo packs etc).
This method will hopefully gain level faster (overall) in my first common character (which I plan to focus on leveling to 20), which in turn will also help unlock the Bonus stats for all characters.
Once I hit 20, I can choose between continuing to play with my lvl 20 Character or start a new Character class. Because different Bonus Stats are only unlocked using different Character classes.
Occasionally buy an Advanced or higher Supply pack to unlock Uncommon or higher character cards / weapons / mods.
Once satisfied with Common characters and weapons being unlocked, then I can focus on higher tier Supply Packs.
The devs claimed that once we unlocked Character / Weapon / Mod cards of 10, we will not receive those again. So no repeatables once a Character / Weapon / Mod is fully unlocked, but since there is so many to unlock, it will still take us a while.
Plus there are other distractions like time-limited rewards via Mission Funds etc.