Flak is a Black Hole CO who can produce wildly different outcomes on any given attack, sometimes in his favor, but also possibly backfiring.
Flak will play like any standard, non-specialized CO, but with damage values being much more inconsistent. Flak can deal heavy damage to vehicles with infantry, but is also just as likely to fail at killing units in otherwise guaranteed matchups, such as Anti-Airs attacking Infantry. Due to his unwieldy nature, Flak generally benefits from having inexpensive units attacking more expensive units, as he will either roll High luck and make highly cost effective attacks, or if he rolls Low luck, will not suffer as greatly due to his units being easily replaceable.
As Luck ranges diminish as a unit's HP is depleted alongside their general combat effectiveness, Flak benefits highly from keeping his units in healthy condition compared to other COs. Flak also generally benefits from having a high unit count as every additional unit is an additional luck roll, with an increased chance of getting a highly damaging attack.
For the most part, playing against Flak is similar to playing against any average CO, with the only difference being the increased need to protect your more valuable units due to the threat of Flak getting any potential lucky shots in. Since Flak's effectiveness greatly deteriorates the more damage his units have sustained, dealing heavy damage through first strike is important against him.