In M:AA there's both solo play and PVP and while both have a lot of things in common, there are two things in PVP that are a little different from the normal gameplay: Armory and Hero Bonus.
PVP Armory
Easy enough, something is better than nothing, so try to have all of your Armory slots filled. Last Spec Ops supplies are usually the best non gold option (even at 720 Shield Points and 144000 Silver once you have 6 pages).
http://i.imgur.com/CoUUntx.png
OFF improves Attack, Accuracy and Stamina.
DEF improves Defense, Evasion and Health.
Hero Bonus
This one can be a little trickier. Yes, it's better to have more heroes and yes, it's better if they are at a higher level.
But, different heroes improve different skills depending on their class and usually only one at a time (except Tacticians (2) and Generalists (All)), so you have to decide what you want to prioritize and may want to train your heroes accordingly.
Those bonuses are:
http://i.imgur.com/TeNa2Ds.png
- Tactician: Health & Stamina
- Blaster: Attack
- Bruiser: Defense
- Scrapper: Accuracy
- Infiltrator: Evasion
- Generalist: All Stats
How much of a bonus you get depends on 3 things:
How many heroes you have on that class .
What level said heroes are at.
Your agent's level.
While the number of heroes and their level affect only their stat, your agents' affects all of the bonuses.
Number of heroes:
Easy enough, more heroes more bonus.
Hero Level:
This one has 2 parts:
The first one starting from level 1, the hero will get a bonus which increases by exactly the same amount each level. Since higher levels take longer, this means that training lower level heroes is more effective that training higher level heroes.
http://i.imgur.com/11FQK9Y.png
The second part appears starting with level 10. This is the elite bonus. As with the hero bonus it increases each level, but it's not lineal, it increases more from level 12 to 13 than it did from 10 to 11. There are 5 different levels of Elite bonus: Bronze (lvl 10), Silver (lvl 11), Gold (lvl 12), Diamond (lvl 13) and Vibranium (lvl 14). Other than recruiting a new hero this are the biggest bonus increases you can get.
Agent Level
As previously stated, this affects all of your heroes, but as opposed to hero levels it increases the same for everyone each time your agent levels up. This means that a level 200 agent with the same heroes at the same level as a level 50 agent will have a better bonus. And while the level and elite bonuses depend on both agent and hero level, the class bonus is only dependant on this stat.
Final thoughts
As tempting as some alts are, it's better to recruit new heroes until you have a sizeable amount.
Unless there's a hero you absolutely want to have i'd recommend to go for the cheapest ones first. Most of them are OK and there are a couple of great ones.
You will get Silver in many ways but the best way to get it is by doing 20 min flights non-stop. Second best is to sell stuff you don't need/want.
The best way to get Shield Points is to visit ally's maps. Second best is on the facebook newsfeed.
You will get exp for your heroes by fighting, but since you only get 2 heroes at a time into battle (and, because of team ups, sometimes only one), short flights (10 or 20 min) are the most consistant way you'll get it.
ThorWolvieGambit has a blog with some cheap team suggestions