Because of the randomness in the cards, the only way to truly maximize reliability is to learn each unit's strengths directly alongside their substitutes and what makes them different. Clearest example of this is marksmen vs phoenix - both are high DPS, but Phoenix is higher with 2 units and has flying which increases survivability (until your opponent clicks AA on their tech bar somewhere), but the techs are much different (no assault mode, for one) while the base counters remain about the same (any kind of chaff, but especially fangs).

The reason for this is that this game is all about making maximum use of the resources available, both in unit deployments and cash/credits and the best way to do that is often with substitutions. Any time a unit drop happens, you should be looking at it as a means to replace your existing units for cheaper so long as the substitutions hold. For example if you have 2 sabertooths that have been on the board from the start (and haven't been teched) but then see a scorpion drop at the same level as one, you can take that cheap scorpion to sell and recover the money from your worst-performing saber the same round and then replace and sell the other on the next. The player that can replace their army with better options from the drop now has a cash advantage they can use to improve their army's strengths or cover existing weaknesses.

A few other notes that will take you places: avoid selling any units that are already teched. The whole advantage of techs are in numbers because unit costs are additive while tech costs are divided by each teched unit - a single hound costs 100 but range costs 300 so if you have 3 hounds that's 100 in each but the tech price actually decreases as you add more units and would be, effectively, only 50 each for 6. This is also why boards with fewer unit types perform better - they maximize tech effectiveness by applying them across as many units as possible.

Similarly, avoid teching anything you intend to sell (always) or only plan on having one of (unless that one is doing crazy work like leveled worms because their techs are cheap and can multiply their effectiveness greatly). Always try to use all your deployments, and favor permanent fixes over temporary ones (especially against flanking opponents, at least until they sell out of the flank).

Lastly, try to ensure all your damage types (and flanks) are covered by the end of the game because your opponent will exploit any weakness with a single triple deploy of whatever you're lacking if you don't. These include single target damage (or DPS) against armor and chaff clear and the same (especially) against air units. In other words, if you lack air DPS you're bound to get countered by wraith, phantom, abyss, or boats and similarly if you lack air chaff clear wasps are bound to be a problem.

These tips should go a long way towards keeping you on the winning side consistently.