The idea is to storm Warsaw with 4 tank, 1 corps and 1 tac-bomber attacks to kill that garrison. This strategy heavily employs the rule that units with less than 5 strength do not radiate ZOCs.
Red arrows indicates attacks; blue arrows indicates movements; yellow arrows indicates bombards.
1: attack bordering Polish units to reduce their strength to less than 5
2: use the fighter recon to trigger interception; advancing units
3: destroy the southern cavalry (the attack is usually enough, if not, use the tac-bomber); bombard Poznan prior to an assault to reduce its strength
4: assualt Poznan. This army attacking is usually enough. However, in my case it did not inflict damage.
5: bringing in another army to attack poznan (as red circle shows). Now there is nothing stopping our two tanks from reaching Warsaw
6: reduce the Polish Corps' strength so that the army could reach Warsaw. Assualt Warsaw. The tac-bomber should be used after 2-4 land attacks so that it could inflict more strength dmg as the entrenchment is lower. 5 land attacks an a tac-bomber attack is almost certain enough to destroy that garrison.
7: the army has 1 action point left. Warsaw is yours
