The Summoner

The Summoner is the main enemy of level 4 (Collector's Flying Saucer) and the final boss in stage 24, The Summoner's Stage.

Strategy
The Summoner is immune to physical damage. Replace all warrior type weapons with gunslinger and engineer types. Focus on either a team that primarily centers on either paralyzing, resurrection, or lastly raw and pure damage if a capable team is available. Diversification does not provide an advantage here, but including an offensive Roger in the former two recommendations is greatly suggested nevertheless. Ensure offensive Rogers' have the highest health here.

Immediately use a mutagen at the beginning of the fight to reset the power delay and immediately use a healing power afterwards. Nano shields should not be used as tanks are not recommended in this fight.

Don't give up after one attempt. Although unlikely, the frequent changes in RNG may result in enough luck to pass the stage without needing to spend hours upgrading Rogers' or a change in strategy.

If The Summoner was not beaten after many attempts using varying strategies then leveling of Rogers' and acquiring stronger weapons is required. This stage can easily be won when a fifth Roger space is unlocked.