People mix up different bonuses:
You get a 15% stackable flanking bonus for every additional unit of yours that is located next to the enemy unit you are attacking. There's six sizes to the hex, so your unit can attack from one side and get backup from 5 others. That's up to +75% bonus from flanking.
If you unlock the honor tree policy, you also get a one-time non-stackable 15% bonus if you have any other of your own units located next to the unit you're attacking with. The other unit could be behind your attacking unit, it doesn't have to be near the enemy.
Neither of these bonuses work on cities.
Stack these up and you get get up to +90% bonuses just from pure placement.
Add in a +25% from a great general from up to two squares away (+45% for Chinese).
Add up to +65%/75% from promotions matched to terrain type (Shock, Drill).
Add a -33% modifier for the enemy if you can catch him in an open field.
And up to +83% from friendly territory bonuses if you get himeji castle, nationalism & oligarchy.
Total War can also net you a 33% bonus for 20 turns.
Now we're at up to about +325%.
A base spearman (7+325%=30) with all those bonuses wouldn't stand even with a tank (50), but it wouldn't be a total pushover. A spearman UU like a Hoplite or Landsknecht would get pretty close though.
Spice it up with some Ambush promotions for an extra +50% and that spearman will totally send that tank packing.